EVERY POISONED RUSSIAN DISSIDENT, every armed guard, every hospital — in fact, every object in the world and beyond it — is made of atoms.

At the heart of every atom sits the nucleus, a tight ball of protons and neutrons. In some ways it is a place of calm. An atom of helium can cross the vast reaches of space with its nucleus intact; carbon nuclei can rest calmly within rocks while civilizations rise and fall. But there are other, less stable, kinds of atoms. Add just a couple of neutrons to a carbon nucleus, for example, and the element will shed its placid nature. Out of balance, it will jettison the unwanted material in order to find stability. We call that emission radiation.

When someone says an atom is “heavy”, they mean that a large number of protons and neutrons are packed into its nucleus. And when a heavy element loses its balance, its search for stability can have dramatic consequences.

Uranium, one of the heaviest elements that occurs naturally, is a lumbering hulk. In its most common form, it has 238 particles crammed into the nucleus. And in its quest to find balance it spits out a lot of material.

As it throws out these chunks — cannonballs containing two protons and two neutrons, a combination known as an alpha particle — it cascades down the periodic table, transforming itself into a different element each time. Just before its arrival at lead-206, it becomes a substance called polonium-210. And it is at this point that the elements of science become the elements of murder.

IT WASN’T UNTIL AFTER she’d moved in with him that Marina saw the stranger, darker side of Alexander Litvinenko. It was a moment she’d never forget.

Marina was a dance teacher. Petite, with short, bobbed hair, small eyes and high, prominent cheeks that tapered into a button chin, she’d fallen in love with Litvinenko after meeting him at her 31st birthday party in the summer of 1993. Even from the beginning, he surprised her. He was an officer with the Federal Security Service, the FSB, and had been an agent for its dreaded Soviet predecessor, the KGB. Even so, there was an unusual lightness about him. He’d bring flowers to her doorstep, or arrive unexpectedly with a bag of bananas, which she’d once told him she loved. He was passionate and loyal. He was, she would one day write, “as emotional as a child”.

A few months after they became a couple, though, Marina glimpsed a version of Alexander which he’d kept hidden.

She’d been taking driving lessons, and as the course came to an end, a traffic patrolman told the group that if they wanted an easy pass, all they had to do was to pay $200, “for the cops”. Marina decided to take the test instead. She was a good driver. She neither needed nor wanted to bribe anyone. But the traffic patrolman failed her on purpose: honesty wasn’t an option. Then he raised the price to $300.

Alexander was incensed. “Do you really think that I fight corruption day and night for you to pay bribes to these cops?” he growled. They went to visit the patrolman, and Marina watched Alexander speak quietly to him. He flashed his red FSB card and, just for an instant, Alexander’s face took on a dangerous quality. The patrolman immediately offered to pass her, no fee required. But that made Alexander even angrier. The patrolman would test her properly, and pass Marina only if she earned that right. As soon as this was agreed, and the test was administered, the Alexander she knew returned, smiling, patting the patrolman on the back. The secret, dangerous doppelgänger vanished.

Litvinenko’s passion began in boyhood. Born in 1962, he was brought up in Nalchik, a city in the remote foothills of the Caucasus mountains, by his paternal grandfather, a world war two fighter pilot. The old man would tell his grandson patriotic stories about entire classes of Russian schoolboys that rose as one to fight the invading Nazis. When he was five, Litvinenko’s grandfather took him to a local museum and showed him a Red Army banner that bore the insignia of the old man’s regiment. “All of our family have defended Russia,” the old man told the boy. “You will too.”

And so he did. Litvinenko joined the army at seventeen. He thrived, and before long he was invited to join the KGB. His adversary was organized crime, and as an ‘oper’ he fought it by studying it like a spy: gathering evidence, analyzing structures and networks, and investigating links with police, businessmen, and politicians.

His mission of simple patriotism became more complex as the Soviet Union dissolved and became the Russia Federation in 1991. During the chaotic years that followed, Russia’s political leaders morphed the state-run economy into something new: a rampant, privatized system that was swiftly dominated by a small group of well-connected individuals. Using their influence and ruthless capitalist instincts, this group gained control of many of the nation’s most valuable enterprises. They became known as the oligarchs.

They knew that spending tens of millions of dollars to acquire stakes in former state-owned enterprises had the potential to make them billionaires, but it was a risky strategy. To hit the jackpot they needed a sequence of unlikely events to happen. First Boris Yeltsin had to remain in power to prevent the Communists from renationalizing their assets. Then the gangsters who dominated many of these industries had to be removed. And then, finally, they needed the Russian economy to bloom.

This epic gamble paid off for the oligarchs, but now they had to hang on to their money and the power it brought. And it was this stage of the extreme stakes poker game that eventually pulled in Litvinenko, as he found himself dragged into the orbit of a dark-eyed, bearish, charismatic oligarch named Boris Berezovsky.

IF IT BEGINS ANYWHERE, the tale of Litvinenko’s fatal disillusionment with his country starts on March 2nd, 1995. That morning, Litvinenko was briefing his FSB bosses on the activities of a murderous mafia group known as the Kurgan gang. The mobsters were linked to a string of executions and attempted killings, including one attack on Boris Berezovsky, an ambitious businessman who was rising rapidly to prominence. And they had infiltrated parts of the Moscow police department, operating as if they were untouchable.

During the meeting, Litvinenko’s pager buzzed: it was Berezovsky himself. The message had an unusual urgency — the mogul wanted him to “call immediately”. It turned out that eight armed Moscow policeman were at a club used by Berezovsky, trying to arrest him. Berezovsky suspected they were acting on behalf of the Kurgan gang, with orders to kill him. Litvinenko raced to the scene, pulled out his gun and insisted that the FSB was taking the case over.

His arrival saved Berezovsky’s life. The oligarch was so grateful he swore that from then on they would be “like brothers.”

The incident prompted Litvinenko to question the motives of those around him, but he pushed back his doubts and continued his battle against organized crime, obsessively investigating how mafia groups were infiltrating official organizations — even his own FSB. His new friend Berezovsky helped him, securing him meetings with senior bureaucrats and even individuals at the Kremlin. But these meetings only deepened his disillusionment.

“I was so naive,” Litvinenko would later tell Goldfarb. “I thought that since they were the big bosses, they would take care of it and stop the mayhem in the services. Not in the least. Every time the threads led high enough, it turned out that the person involved was somebody’s buddy or relative or comrade in arms. The only thing I achieved is a certain reputation: the village idiot… the whole system was rotten to the core.”

Later that year, Litvinenko’s faith in the state was tested in the first Chechen War. He was a member of the special forces, the “Osobysty”, and believed the narrative that the media had been telling him: the Chechens were terrorists, rapists, evil torturers, animals. But while interrogating one captured fighter, just a teenager, Litvinenko had a conversation that came to haunt him. The boy told Litvinenko that he hated the war, but believed the fighting had to be done: everyone else in his class had done the same.

Litvinenko remembered the stories his grandfather told, of Russian children signing up to fight as German armies drew ever closer. “Entire classes of schoolboys don’t join terrorist organizations,” he realized. That was when he decided: his bosses, whom he trusted and frequently risked his life for, had lied to him. And, as Marina could have told them, Alexander always found it hard to excuse betrayal.

§

DISCOVERED BY FRENCH SCIENTISTS Pierre and Marie Curie in the late 19th century and named after Marie’s home country, Poland, radioactive polonium-210 exists naturally in the environment. It’s in tobacco leaves and shellfish. It’s even inside human beings, in extremely small amounts.

Radiation, whatever its source and whatever its type, can damage the DNA in our cells as it passes through us. But the body knows how to fix DNA: in fact, it’s busy with that job every second that we’re alive. “We’ve been sitting in a sea of radiation throughout our evolution,” says Richard Wakeford, an epidemiologist at the University of Manchester. “It’s been higher in the past than it is now. Life would not exist without very effective DNA repair mechanisms.” Sometimes, however, the damage is so profound that the DNA cannot be repaired. When this happens, says Wakeford, “the cell commits suicide, essentially.”

Polonium-210 spits out alpha particles. Normally, alphas aren’t a problem. Their huge size means that even a barrier as thin as a few inches of air — or a layer of human skin — stops them before they can do any harm. But if a high dose finds its way inside a human body, the damage can be immense. “An alpha particle leaves the equivalent of a motorway as it goes through tissue,” says Wakeford. “Sufficient cells in your body would commit suicide that you’d die.”

Polonium-210 is unusual in another way, too: it leaves the body relatively quickly. It sheds radiation so fast that the amount of poison in the body decreases by half in less than two months. “It’s sufficiently long lived that it’ll be around for a bit, but after that will leave no trace,” says Wakeford.

“Polonium-210 is very rare in that it is almost a pure alpha emitter. When it gives out alpha particles there’s hardly any chance of there being a gamma particle emitted as well. You need a special instrument to look for alpha particles, because they’re so easy to block. ”

In short, it’s deadly, hard to find and doesn’t hang around. “And that,” says Wakeford, “is why it makes such a good poison.”

§

FROM THE BLOOD and secrets of the first Chechen war, the FSB took a dark lesson; extraordinary circumstances sometimes require action outside the law. The decision was taken to form a secret division to deal with criminals decisively, lethally if necessary. They called it the Division of Operations Against Criminal Organizations, or URPO. It was a government-sanctioned assassination team, staffed by 40 or so especially ruthless FSB opers. One of them was Alexander Litvinenko.

In December 1997, his boss, URPO deputy chief Alexander Kamyshnikov, called Litvinenko and three other men into his office. Because of his special access, he was given a stunning new mission: to murder his friend Berezovsky.

None of the URPO men wanted to carry out the assassination. They persuaded themselves that Kamyshnikov must have been acting under his own volition. Berezovsky was a famous and successful businessman, owner of the nation’s biggest TV broadcaster, Russian Public TV, and until recently had been the deputy head of the government’s national security council. Surely their bosses couldn’t know about this?

Litvinenko and his URPO colleagues decided to write a report. After it was presented to FSB director Nikolai Kovalyov, all the opers involved were promptly suspended from duty. It was a difficult time for them: Litvinenko had trouble sleeping and started losing weight. Eventually, the stress led him to two decisions that would once have seemed unthinkable.

First, in March 1998, he walked into a meeting with Berezovsky and betrayed the FSB. Choosing his “brother” over his country, he explained what had happened. “Boris, my superiors told me I should kill you.” At first Berezovsky thought it was a joke.

Then, a few months later, Litvinenko and his URPO whistleblowers staged a television press conference in front of the Russian media. Unlike three of his colleagues, Litvinenko chose not to wear a disguise, preferring to present himself as a leader on the side of good. With the cameras pointing at him, wearing a brown suit with wide lapels, a swirly colorful tie, hair brushed and eyelids heavy and dark, Litvinenko began reading from a prepared statement.

“We would like to emphasize right off that we are not the enemies of the FSB,” he said. “We do not seek to compromise the Federal Security Service, but to purify and strengthen it.”

However, he continued, “The FSB is being used by certain officials solely for their private purposes. Instead of its original constitutional aim of providing security for the state and for citizens, it’s now being used for settling scores and carrying out private and criminal orders for payments.”

The broadcast was a sensation.

Since his death, this sequence of events has been portrayed by Litvinenko’s widow Marina and friend Alex Goldfarb as an act of simple yet extraordinary loyalty by an essentially blameless man.

Litvinenko, they say in their book Death of a Dissident, told Berezovsky about the assassination order because that order was ideologically motivated and immoral. Litvinenko’s reaction was emotional, boy-like — he didn’t want to commit any sort of murder, let alone execute his friend. And, they argue, no inference should be drawn from the fact that he was a member of the feared Osobysty during a notorious war. Nor was it significant that recruits to the secretive and ruthless URPO were drawn from the FSB’s most brutal quarters.

“I don’t say I’m an angel,” he once said, “but I don’t have blood on my hands.”

But can this version of Litvinenko, and the allegations he made, be real? Martin Sixsmith, the BBC’s former Moscow correspondent, believes Litvinenko’s reaction to being recruited into the sinister URPO was pure pride. “He told Marina it was a great honour to be selected,” Sixsmith writes in The Litvinenko File. “He seems to have had no qualms about the nature of the work he would be doing.”

Litvinenko told friends that he was selected because he once defended subordinates who had used a firearm illegally in court, and because of his own reputation as a tough guy. “There are persistent stories about [his] life that cast doubt on his widow’s idealized portrait of him,” says Sixsmith. “This explanation sounds like Litvinenko was being a little economical with the truth about his own ruthlessness.” Sixsmith believes Litvinenko was involved in killings in Chechnya, for example. There were even rumours, disputed by many as Kremlin propaganda, that he was known for torturing prisoners in his care.

Others have a more pragmatic view why he took a stand against the FSB.

Yuri Felshtinsky first met Litvinenko in November 1998, when he helped edit the URPO press statement at Berezovsky’s villa. The two became friends and would go on to write the book Blowing Up Russia together. According to Felshtinsky, Litvinenko, like the oligarchs, was gambling in his own self-interest.

Throughout this period, Berezovsky’s political power was immense. As well as his influence over Boris Yeltsin and his prominent role in the media, Berezovsky was close friends with Litvinenko’s boss, the newly appointed FSB director. At that time, Litvinenko’s “brother” was widely perceived to have been the most powerful man in Russia. If Berezovsky used his influence to sweep out the old leadership of the FSB, Litvinenko believed his bet would pay off, says Felshtinsky.

But Litvinenko had misjudged the new director of the FSB: Vladimir Putin. The two had met before the press conference, and Litvinenko had urged him to start reducing corruption inside the services. “I could see it in his eyes that he hated me,” Litvinenko said after the encounter. He was the FSB’s man “body and soul. And, to him, I am a traitor.”

After the press conference, Putin ridiculed the whistleblowers — and then he began going after Litvinenko. According to Felshtinsky, this came as a shock. Litvinenko was so convinced of Berezovsky’s power that he thought he’d be protected, even after the unpleasant meeting with Putin. That misjudgement ran so deep he was actually expecting to be promoted. “Litvinenko came to my apartment in Moscow and whispered to me that he has very reliable information that tomorrow he will be promoted to a major position within the FSB,” Felshtinsky told me. “Well, the next day he was promoted… to Lefortovo prison.”

Having been arrested on the charge of “exceeding his official powers” and causing actual bodily harm, Litvinenko spent seven months in Lefortovo, notorious for housing political prisoners during the height of the KGB’s powers. In October 1999 he was found not guilty, but before he had even left the court, he was re-arrested by the FSB on charges of further violence — and, bizarrely, stealing a can of peas. Television footage of this his re-arrest caused outrage, and Putin was forced to apologize. But while those charges were eventually thrown out, they were followed by even more.

“In the KGB code, the punishment for traitors is you could be killed,” explains Luke Harding, a former Moscow correspondent for The Guardian. “And that’s how Putin regarded him. As a traitor.”

By now, Litvinenko knew he had to escape, and in October 2000, he fled to Turkey. From there, with the help of Berezovsky and Goldfarb, he landed in London shortly afterwards.

Berezovsky too had also started feeling Putin’s displeasure. Elected president in 2000, Putin was the leader of a new faction: men disgusted by the sight of the nation’s riches being shared out between Berezovsky and the others — because they wanted the cash for themselves.

“The thing you have to understand is, it’s all about money,” says Harding, who was himself barred from Russia for reporting critically on the Putin regime. “None of the differences between these factions is ideological.”

In the years that followed his election, Putin’s people would gain control of state assets and become multi-billionaires. They would also come into conflict with the original oligarchs, including Berezovsky. Litvinenko’s friend Felshtinsky says, “Knowing Berezovsky for many years, I have to tell you he sometimes has difficulties to understand the reality. He’s a dreamer. He has this idea that he’s so powerful that he could force the president down if he wants to.”

In fact, it was a struggle that Berezovsky could not win. “He was competing with Putin for political power,” says Felshtinsky. “And that’s when he had to be destroyed.”

In October 2000, a journalist from Le Figaro asked Putin about Berezovsky. “The state has a cudgel in its hands that you use to hit just once, but on the head,” the president said. “We haven’t used this cudgel yet… the day we get really angry, we won’t hesitate to use it.”

With the oligarch’s support, Litvinenko spent the next few years investigating and fighting the FSB the same way that he used to investigate and fight organized crime. He was, Harding says, an aggressive, confrontational campaigner.

In 2002, Felshtinsky and Litvinenko published Blowing Up Russia. In it, they accused the FSB of orchestrating terrorist bombs in Moscow in 1999, so-called false-flag atrocities that were used to justify the start of a second Chechen war and, in theory, helped Putin win the presidency. The publication was an attempt at revenge. Goldfarb wrote: “For them, this book was not an act aimed at the general public. It was a personal message to their nemeses, a declaration of war.”

This crusade was not always clear-eyed. In the final years of Litvinenko’s life, the charges he made against his enemies became increasingly paranoid. He accused Putin of being a paedophile, and the FSB of training Osama bin Laden’s deputy prior to the New York attacks of September 2001. Perhaps this is what happens when you get close enough to true power to glimpse its size and force. To have made such an enemy must have been dizzying, awful — and thrilling. His fight against his foes seems to have intoxicated Litvinenko. Maybe it affected his judgment.

Still, on June 8th, 2006, the State Duma passed new legislation that provided for “special operations units of the FSB to be used at the discretion of the President against terrorists and bases that are located outside the Russian Federation for the purpose of interdicting threats to the Russian Federation.”

Five months later, Litvinenko hopped on the 134 bus towards Grosvenor Square. He was headed for the Pine Bar.

BECAUSE IT IS SO HIGHLY SOLUBLE, polonium-210 is easily ingested. And when Litvinenko started vomiting on the evening of November 1st, the radiation had already begun to destroy the lining of his gut.

The cells lining the walls of the stomach are among the first to react to the toxin. They start sloughing and breaking away minutes after contact. The intestines, and the soft, unprotected skin inside the throat and mouth suffer the same fate.

Polonium is hugely radioactive, firing off a massive bombardment of alpha particles — and without any screening, the delicate mechanisms of the body’s internal organs get the full dose. As the atoms try to stabilize, alpha particles crash into nearby body tissue, knocking electrons from the molecules they encounter. Each time they do, the trail of wrecked cells expands; the poison turns them cancerous, or kills them off entirely.

And that is just the beginning. Even as it weakens, the stomach continues to digests its contents, pulling the polonium into the bloodstream. Once it’s there, the poison uses the body’s own functions against it: each beat of the heart distributes radioactive material further around the organs and soft tissues.

As the blood unwittingly delivers its payload, polonium gathers in its most perilous quantities in the liver, spleen, bone marrow, kidneys, skin, and hair follicles. From there it emits alpha particles at a devastating rate. Motorways of dying cells are pushed out through the body.

The sickness makes itself visible on the outside. Alongside the nausea and vomiting comes headache, fever, and diarrhoea: victims weaken, and their skin may develop blotchy red spots. Like Litvinenko, their hair will drop out. At high doses, they will get dizzy and fall over as their blood pressure drops.

All of these can debilitate or incapacitate, but it is when the poison arrives in the red bone marrow that death begins.

By the time it invades the ends of the long bones and gets into the marrow, the terror has reached its most vital target — the place where new blood cells are created. Just as skin cells were mutilated as the alpha particles smashed their way through, the marrow cells are stripped of their electrons and left corrupted, damaged, or dead.

With its source of new blood polluted by radioactivity, the body goes haywire. The lymphatic system begins to shut down. White blood cell count drops. The body loses its ability to fight off disease. It commits suicide, cell by cell.

“Once that happened,” says Wakeford, of the bone marrow contamination, “Litvinenko was finished. In fact, he was finished when he took that swallow of tea. There was nothing that could have been done for him. He was a dead man from that moment on. It was amazing he lasted as long as he did.”

Weight-for-weight, polonium-210 is around 250,000 times more toxic than hydrogen cyanide, the favored poison of the Nazis during World War II.

It has industrial uses, for example in eliminators used to remove static cling in factories producing paper or synthetic fibres. But it is far from easily available. The amount ingested by Litvinenko is thought to have been less than a microgram — a speck of dust. It was a tiny amount, but many times more than needed to kill him. Because polonium-210 has such a short half-life, that much of the substance would have to have been newly created. Which means whoever killed Litvinenko had access to a nuclear reactor.

§

THE IDEA OF POISONING — radioactive or otherwise — is not new to Russian intelligence. According to former Russian intelligence officer Boris Volodarsky, now a historian and one-time associate of Litvinenko, the Russians have a history of substance assassination going back nearly a century. It was Lenin who ordered the establishment of their first laboratory, known simply as the ‘Special Room’, for developing new lethal toxins.

“There is also a long succession of poisonings by Russian intelligence services in different countries, starting in the early 1920s,” he says.

At its height, says Volodarsky, the Soviet Union had the largest biological warfare program in the world. Sources have claimed there were 40,000 individuals, including 9,000 scientists, working at 47 different facilities. More than 1,000 of these experts specialized in the development and application of deadly compounds. They used lethal gasses, skin contact poisons that were smeared on door handles and nerve toxins said to be untraceable. The idea, at all times, was to make death seem natural — or, at the very least, to confuse doctors and investigators. “It’s never designed to demonstrate anything, only to kill the victim, quietly and unobtrusively,” Volodarsky writes in The KGB’s Poison Factory. “This was an unbreakable principle.”

Murderous poisons come in three varieties: chemical, biological, and radiological. It’s believed that the first Soviet attempt at a radiological assassination took place in 1957. The target was Nikolai Khokhlov, a defector who had left for the United States a few years earlier. He became drastically ill after drinking coffee at an anti-communist conference he was speaking at in West Germany. After his collapse, he was successfully treated at a US army hospital in Frankfurt for what was believed to be poisoning by radioactive thallium.

In the years before Litvinenko’s murder, a series of other killings bore similar hallmarks. In 2004, Roman Tsepov, a prominent and controversial political operative from St Petersburg, died after being poisoned with an unidentified radioactive substance on a visit to Moscow. The year before, Yuri Shchekochikhin had died in similarly mysterious circumstances. An investigative journalist and member of parliament, he had exposed a series of scandals, including an FSB racket that laundered money through the Bank of New York. His death came after a brief, undiagnosed illness with familiar symptoms: hair loss, vomiting, red blotches, fatigue. He was due to fly to the United States to meet FBI agents just days later.

The Russians may not be the only ones to have used nuclear technology for targeted execution, however. East Germany’s secret police, the Stasi, are alleged to have used radioactive poisons and even deployed modified x-ray machines to irradiate and injure political prisoners. They also used radiation as a tool, surreptitiously tagging dissidents with chemicals so they could trace and track them with Geiger counters.

And in November 2013, scientists in Switzerland announced that they had found heightened levels of polonium in the remains of the former Palestinian leader, Yasser Arafat. Investigations began two years previously, when researchers discovered the strange news that some of the personal items that Arafat had been wearing shortly before his death appeared to be contaminated with high levels of polonium-210 and were emitting alpha radiation. It was a finding that raised difficult questions for those who may have wanted to get rid of him.

But still, Russia’s ability to source and use radioactive poisons seems to be pre-eminent. Only about 100 grams of polonium are manufactured each year, and just three countries are known to produce it reliably: Israel, the United States, and Russia. And 97 percent of that supply is manufactured in one place — a converted nuclear weapons facility that operates under high security, on the banks of the Volga, 450 miles south east of the Kremlin.

The case for official Russian involvement in Litvinenko’s death was growing. And there was more evidence to come: unbeknown to them, the assassins had left a trail — and it seemed to lead east.

IT’S UNCLEAR EXACTLY who first came up with the idea that Litvinenko’s doctors should check for such a rare and unlikely poison as polonium-210. Most think it came in a brilliant flash of inspiration by John Henry, the toxicologist that Berezovsky’s team had brought in. However, Wakeford offers an intriguing alternative: “I’ve heard that some Russians said ‘this sounds like polonium to us,’ because they were familiar with the symptoms.”

Wherever the tip off came from, Litvinenko’s urine — one of the easiest places to test — was checked by the doctors as University College Hospital. “In one in 100,000 decays of polonium-210 a low energy gamma particle is emitted,” says Wakeford. “And he had such a tremendous amount inside him that they found some.”

The discovery of radioactive poison triggered an immediate response at Britain’s Health Protection Agency. Even while Litvinenko was alive, Roger Cox, the director of the agency’s Centre for Radiation, Chemical & Environmental Hazards, had begun covertly recruiting a squad of experts to deal with what he suspected might be a political assassination. And as soon as polonium-210 had been confirmed as the lethal agent, his team calculated the likely size of Litvinenko’s dose and its potential for toxic contamination of other people and places.

Polonium is devastating if ingested, inhaled, or absorbed, but it cannot penetrate human skin. That meant the chances of cross-contamination were very small, but with such a lethal element no risks could be taken. On the night of Litvinenko’s death, Cox phoned his boss, Professor Pat Troop, the agency’s chief executive, who was waiting for news while on a trip to Helsinki.

The next morning, she briefed government ministers and the media. Then her team got to work.

The first step was to send out forensic investigators with protective suits, who used sensitive probes to detect the radiation trail that let them retrace Litvinenko’s steps. And they found it almost everywhere they looked: at the Pine Bar, in the hotel’s kitchens, at the offices of Berezovsky, where he’d stopped before going home, and in the Mercedes he’d sat in on the way back to Muswell Hill.

The list of contaminated locations grew rapidly, soon swelling to at least 50 in London alone. But following the trail was not easy. Because alpha radiation is so heavy and short-ranged, investigators had to crawl on hands and knees, holding their probes between two and three centimeters from a suspected source. Each discovery was followed by decontamination: solid surfaces were cleaned with wet wipes and Decon-90, a powerful detergent that contains potassium hydroxide, potent enough to break down and disinfect anything touched by polonium. Wooden surfaces were varnished over, walls coated in thick paint that trapped the radiation beneath it until it decayed into safer form. Baths and basins whose porous enamel surfaces had absorbed polonium-210 were smashed with hammers, and the pieces were bagged and taken away as radioactive waste.

As forensic investigators followed the potentially deadly trail, they found themselves criss-crossing London. The clean-up to make the Millennium Hotel completely safe took 19 days from start to finish: in total, the decontamination project employed more than 3,000 people and tested more than 700.

But the polonium trail was more than a public health hazard. The police, too, were following it. The element’s rapid rate of decay made it an attractive poison for the assassin: after all, in a matter of weeks any trace of it would have started to disappear. But the moment the investigating officers were told what had killed Litvinenko, that high-energy decay became a beacon instead. Because it fades rapidly with time, and changes depending on whether the polonium-210 was handled directly or ingested and expelled in sweat, the chronology of the trail could be recreated. All the police had to do was measure the strength of the alpha emissions. Instead of keeping the killers’ identities secret, the poison had created a gigantic flashing arrow, pointing directly at them.

The investigation was led by Peter Clarke of the Counter Terrorism Command, an experienced officer who had investigated the bombings of July 7th, 2005. His team realized that there would be at least two trails of radioactivity to follow: one from Litvinenko as he travelled home, and another from whoever killed him. They found no trace of radiation on the bus Litvinenko took to the hotel, suggesting that he was poisoned only after he arrived. It was, however, all over the picture which he’d sat under in the Pine Bar, as well as on his chair and on the floor nearby. The arc of radioactivity, along with the evidence provided by head barman Norberto Andrade, suggested that the poison had been sprayed into the teapot.

The investigators also used the contamination trails to travel backwards through time and track the movements of Litvinenko’s bar companions in the weeks leading up to the killing. Dmitry Kovtun and Andrei Lugovoy, 41 and 40 at the time of the incident, were childhood neighbors who’d both gone on to serve in the KGB. They’d visited the British capital for a few days on October 16th, a fortnight before the poisoning; the rooms the men had stayed in at the discreet Parkes Hotel, set in a Knightsbridge townhouse, were radioactive. Officers tracked down the aircraft they arrived on, but there the trail went cold: the Russian airline, Transaero, refused to allow them on board. Polonium-210 was also discovered on the British Airways jet that Lugovoy had flown on when he returned to London on October 25th.

The discoveries weren’t always mere traces. Lugovoy’s eighth floor room at The Sheraton on Park Lane had alpha radiation readings so high that it had to be sealed for two months until it was totally clean, and later guests tracked down, tested, and cleared.

While Lugovoy was spreading radiation around his room, Dmitry Kovtun was travelling from Moscow to Hamburg on an Aeroflot plane. Again, Russian authorities refused to permit the British to examine the aircraft. German police, however, discovered that the Hamburg flat at which he’d stayed was contaminated. During his time there, he left traces on everything he touched.

On the eve of the poisoning, the pair once again flew from Moscow to London with British Airways. That aeroplane tested positive for polonium-210 too. After arriving, Lugovoy had taken the opportunity to watch a football match: Arsenal were playing a Champions League match against CSKA Moscow, once the official team of the Soviet military. Investigators checked his ticket number and CCTV footage to track down Lugovoy’s seat: it was so radioactive that it had to be removed and securely burned. HPA investigators were thankful the traces were so small that they did not have to decontaminate the entire 60,000 seat stadium.

Polonium-210 had turned on its masters. But while Litvinenko met with both Kovtun and Lugovoy, he seemed sure which one had been responsible for the actual poisoning.

Andrei Lugovoy is a case study in the unique mode of scheming enterprise so familiar in modern Russia. A man of riches, he holds multi-million dollar investments in the food and service industries and ownership of a successful drinks company. A man of power, he has connections with oligarchs and ran a private security service for fellow Russians of wealth. A man of secrets, ostensibly meeting with Litvinenko to plan a trip to Madrid to pass information to the Spanish secret services about the network of corruption connecting the Russian government, organized crime and European businessmen.

In May 2007, Britain’s Director of Public Prosecutions said he had enough evidence to charge Lugovoy with the murder of Alexander Litvinenko. Safe in Moscow, Lugovoy said he was happy to cooperate with the authorities, but denied the accusations. Instead, he claimed, it was an anti-Russian plot, headed by the UK’s security services. Britain requested his extradition for trial. Russia refused.

§

IN THE TRAIL LEFT BY polonium-210, Litvinenko’s friends recognized not just the fading sparks of radiation. They recognized vengeance, delivered by order. Litvinenko himself believed that Lugovoy was acting under orders of the FSB who were, in turn, instructed by Putin. And he thought he was targeted in large part because of accusations against the president and the agency, especially those contained in Blowing Up Russia.

But there are other theories. Perhaps Litvinenko finally paid for that old, cold enmity; his so-called betrayal of FSB principles. His death could also have resulted from more recent activities connected with the British secret service MI6 — although those remain murky. Or perhaps it was the revelations he was making to the Spanish equivalent, the CNI, which came to light after the Wikileaks diplomatic cable release in November 2010 and subsequent reporting in the Spanish newspaper El Pais.

Says Luke Harding, “He was in Spain in April 2006 and murdered in November. The information he was giving them was very embarrassing for people at the top of the Russian government and directly affects their assets. That’s a strong reason to kill him.”

Whether or not it was carried out by Lugovoy or the FSB, whether or not it was ordered by Putin, whether or not Litvinenko was killed for betrayal, revelation, embarrassment or all three, the shadow of his death remains. And there are several ugly reasons to believe that Britain remains a dangerous haven for those who seek to challenge those with power in Russia.

Since 2006, London and its suburbs have witnessed a series of fatal and near-fatal events, including the shooting of the Russian banker German Gorbuntsov in March 2012, and the unexplained death of 44-year-old financial expert Alexander Perepilichnyy later that year. Perepilichnyy collapsed meters from his home in the pleasant British town of Weybridge, 17 miles south west of London. He had fled Moscow three years earlier, after uncovering a case of fraud that implicated individuals in the Russian tax office, criminal gangs, and police.

Shortly after his death and two inconclusive post-mortems, a former KGB defector told The Guardian about a toxin called sodium fluoride.

“The substance is colorless and without smell. It can be applied to personal items — like a pen, phone or door handle — or to places where the target inhales it. It dissolves in the ‘mark’s’ body. It’s undetectable in any post-mortem carried out.”

Then, on March 23rd 2013, came another brutal twist in Litvinenko’s story: Boris Berezovksy was found dead in a locked bathroom.

There were no signs of struggle and he had bruises on his neck; police said the evidence was consistent with hanging. Berezovksy’s apparent suicide has been explained by his depression and recent bankruptcy following a costly legal battle that he had lost.

Yuri Felshtinsky, who also authored The Putin Corporation, says this simply isn’t true. Berezovsky had actually been short of money for years, he says, but in the months preceding his death the exiled oligarch had actually managed to rescue at least $1 billion of his fortune. And the claims that he was so pained by his legal defeat that he would take his own life? “Trust me,” says Felshtinsky. “Berezovsky was cynical and tough, and he was a survivor.”

Berezovsky left no note. The possibility of murder, said a police spokesperson, “could not be completely eliminated.”