In September 1999 a series of bombings has shaken up the Russian cities of Buinaksk, Moscow and Volgodonsk, destroying several apartment buildings with hundreds of civilians.
The prevailing viewpoint is that the bombings have been perpetrated by separatists from North Caucasus as the act of retribution for Moscow’s military campaign in the republics of Dagestan and Chechnya.  The acts of terrorism were organized by Ibn al Khattab — a Saudi citizen who had the capabilities and the motivation to carry out the bombings. [2, 3]
There is also a conspiracy theory that the bombings have been perpetrated by the Russian Federal Security service — which is called FSB — to make the Russians rally around the flag, with the end effect of boosting the popularity of prime minister Vladimir Putin.
Is there any proof of the theory which blames the Russian security services for the apartment bombings — or is it a mere speculation? The opinions diverge. Former U.S. diplomat Strobe Talbott has asserted that there has been no evidence of the conspiracy theory.  Contrary to that, journalist David Satter believes that while no direct evidence exists, the circumstantial evidence is overwhelming. 
Who is right on that issue? We will try to find out.
To answer that question, we are going to examine some of the circumstantial evidence — and see if it stands the scrutiny.
The real point of contention is the bizarre incident which took place on September 22, 1999 in the city of Ryazan. Some authors — such as Soldatov and Borogan — believe that it has been a routine training exercise performed by the FSB to test the efficacy of counterterrorism measures. And some authors — such as Felshtinsky and Litvinenko — believe that it has been a thwarted attempt by the FSB to explode an apartment block, which by association implies its complicity in the preceding bombings.
1. What happened in Ryazan in September 1999?
Before we proceed, let’s familiarize ourselves with the basic facts of the Ryazan incident.
On September 22, 1999, Alexei Kartofelnikov — a resident of an apartment complex at 14/16 Novoselov Street in Ryazan — spotted a suspicious car near a street door. A part of a license plate was covered with a sheet of paper; two men and a young woman were carrying sacks from the car into the basement of the apartment block. The alarmed citizen called the police, which found three sacks in the basement, that looked like standard 50 kg sugar sacks and contained sugar-like substance. Besides, one of the sacks contained a hunting cartridge used as a detonator, a battery and a timer with the alarm set to 5.30 a.m. [6,7]
The head of the local bomb squad Yuri Tkachenko performed express testing which showed the presence of RDX. However, when experts tried to explode a part of the substance, it did not detonate. 
Nataliya Yukhanova, an employee of a local telephone company, has overheard a suspicious call to Moscow in which the following instruction was given: “Leave one at a time, there are patrols everywhere” — and informed her superiors of it.
On September 23, a news broadcast on the Russian TV reported that the substance found in Ryazan didn’t contain explosives. 
On September 24, head of the Russian Federal Security Service (FSB) Nikolai Patrushev announced that the Ryazan incident was a training exercise, and the device found was a dummy bomb. 
In the aftermath, the local branch of the FSB has awarded Kartofelnikov and Yukhanova with prizes. Actually, that’s how we know about the intercepted phone call — its content was shared by Yukhanova after the award ceremony. 
2. The preceding discussion
Now that we are familiar with the basic facts of the Ryazan incident, let’s have a look at the existing literature. Books which correctly review the facts include Andrew Meier’s “Chechnya: To the Heart of a Conflict” (2004) and Elena Pokalova’s “Chechnya’s Terrorist Network: The Evolution of Terrorism in Russia’s North Caucasus” (2015).
The conspiracy theory outlook was detailed in a book by Yuri Felshtinsky and Alexander Litvinenko, “Blowing Up Russia” (2007). The further discussion will concern the second edition of the book, which was published in 2007. However, the first Russian edition has been published in 2002, and the chapter on the Ryazan incident that is essential to our discussion was published in Russian newspaper “Novaya Gazeta” as early as 2001.
Although the book by Felshtinsky and Litvinenko — and especially its account of the Ryazan incident — is widely cited, the criticism of the conspiracy theory approach to the Ryazan incident remains muted.
In particular, world’s leading independent experts on the Russian security services, Andrei Soldatov and Irina Borogan, have commented thusly: 
“The authors believe that there were indeed exercises carried out in Ryazan. Such exercises are typical for Vympel, a special unit of the FSB with the mission of verifying the efficacy of counterterrorism measures at locales like nuclear plants. But it is also the authors’ impression that the FSB needlessly bungled the crisis by giving an explanation that raised more questions than it answered. The idea that the FSB might have been involved in the bombings to help bring Putin to power became a runaway conspiracy theory. To date, the FSB has failed to counteract this speculation with convincing evidence of what did happen in Ryazan.”
More specific criticism was provided by Dr. Kirill Pankratov, who made the following points in a 2003 article for “Johnson’s Russia List”. First, the FSB — after years of underfunding and neglect — was institutionally incapable of organizing such a large-scale conspiracy. Second, the deliberate sloppiness of FSB agents in Ryazan indicates that they were not really trying to blow up an apartment block — but is consistent with the “training exercise” version of events. And finally, the FSB had a much easier option to avoid any suspicion and controversy — they could just express great relief that the terrorist plot has been foiled, and quietly keep trying to find the perpetrators. 
Still, it remains unclear how should we view the conspiracy theory outlook of the Ryazan incident. Could we address it in a more specific way?
3. A methodology note
Before we start a critical review of someone else’s work, it’s worth asking — is there an even playing field?
Until 1998 Alexander Litvinenko has been an FSB operative with an access to Russia’s state secrets. However, since the scope of this work is limited to the Russian apartment bombings of 1999, with the major emphasis on the Ryazan incident of September 1999, it’s safe assuming that we have the same rights as Litvinenko and Felshtinsky regarding access to the literature.
In the sequel, we presume that the work of Felshtisnky and Litvinenko — as it concerns the Russian apartment bombings and the Ryazan incident — is based solely on open sources. Respectfully, our criticism of their work is also based on open sources. Moreover, all our sources are available on the Internet — and although some are paywalled, one can always visit a library to get them free of charge.
To better get our point across, from now on each section of the article will be ended with a conclusion summarizing the key findings.
4. Were FSB officers detained in Ryazan?
According to Felshtinsky and Litvinenko, two FSB agents were detained in Ryazan on September 24, 1999 by operatives of the local FSB branch — the latter presumed they were investigating a genuine terrorist plot. The news of the impending arrest of the two agents made the head of FSB Nikolai Patrushev make a surprise announcement that the entire incident was actually a training exercise.
Is that information correct? Also, is there any different explanation to the timing of Patrushev’s announcement?
Following the King’s instruction to the White Rabbit, let’s begin at the beginning.
In a May 2000 interview, the head of the investigative section of the Ryazan FSB branch, Yuri Maximov said: [12, 13]
“The announcement about exercises held by the FSB of the Russian Federation came as a complete surprise to us and appeared at a moment when the department of the FSB had identified the places of residence in Ryazan of those involved in planting the dummy (as it subsequently emerged) device and was preparing to detain them.”
Based on the quote by Maximov (and the identical statement made previously by the press office of the Ryazan FSB branch), Felshtinsky and Litvinenko conclude that “Obviously, even though Patrushev had forbidden it, the Ryazan UFSB went ahead and arrested the terrorists”. 
Now let’s tread carefully. The use of the term “obviously” means that that piece of information is an inference, rather than a fact. That inference didn’t appear in any publication preceding Felshtinsky and Litvinenko’s work — which means they have invented it.
Before we proceed — do we have any extra information?
In 2002, details of the FSB operation in Ryazan leaked to the press in an article by Russian journalist Rustam Arifdzhanov. It tells the story of three FSB officers with the first names of Vasily, Pyotr and Tatyana (their last names are not disclosed) sent on a mission by their boss — major general Tikhonov — to test the alertness of Ryazan to a terrorist attack. 
To do that, the FSB officers have purchased sacks of sugar at the city market in Ryazan and put those sacks in the basement of an apartment block.
How did their mission end? “On September 23, Vasily called the Center and was instructed to urgently return to the home base. Vasily, Pyotr and Tatyana have left the city of Ryazan by different routes and in different times. On the morning of September 24, the leader of the group … major Vasily … has written a report to the Center. … The report has immediately gone to major general Tikhonov. He reported to the FSB head. On the noon of September 24, Nikolai Patrushev made a statement about the exercise in Ryazan.”
The behavior of FSB agents described by Arifdzhanov is consistent with the instruction reportedly overheard by Yukhanova: “Leave one at a time”.  Indeed, why would the officers disobey the order?
Now, what could explain the timing of Patrushev’s announcement in view of the article by Arifdzhanov?
Two possible explanations were given at the time of the events. According to Patrushev’s TV interview on September 25, the weak response of the Ryazan law enforcement and security services to the Ryazan incident motivated the FSB to continue the operation to see how effective the local agencies would be to find the perpetrators.  We presume that after the group returned to Moscow there was no reason to continue the operation — although the Ryazan FSB branch could possibly arrest some innocents, that would serve no further goal.
Another, somewhat eerie explanation was given in an article by Stepan Dvornikov and Aleksandr Raskin, published on September 29. Reportedly, the FSB intended to make an announcement of the exercise in Ryazan as soon as it would be discovered that the sacks contained only sugar. However, after the express testing showed the presence of RDX, the FSB leaders in charge of the exercise were genuinely confused about what was going on. What if there’s been some mistake, or their plans have somehow coincided with the plans of actual terrorists? Reportedly, they have calmed down only by the morning of September 23, when more accurate information appeared regarding the testing.  We believe that the safest option in such situation was to wait for the written report by the leader of the group sent to Ryazan.
At this point a new question emerges — how accurate were the results of express testing? That will be discussed in the next section.
The notion that two FSB agents were detained in Ryazan by the local security services is a conjecture — a major leap of faith, unsupported by the evidence. Contrary to that, it’s logical to assume that all three FSB agents have escaped to Moscow on September 23 — as they were instructed by phone.
5. Express analysis of the sugar-like substance
At this point, the only fact that could confirm the theory that the FSB attempted to blow up the apartment block in Ryazan is the result of the express testing which showed the presence of RDX.
Our impression is that it’s also the least understood topic in the literature. Let’s start with reviewing the facts reported in the media.
5.1. The reported facts
The initial media reports (made on September 24, 1999) said that the police bomb squad had used an explosive vapor detector which showed the presence of RDX vapors. 
After a couple weeks, it has been reported that the detector used by the police showed a false positive result due to contamination of the device. The detector should have been washed, but the police lacked the funds to do so. 
Later on, in February 2000, Novaya Gazeta journalist Pavel Voloshin has interviewed the head of the Ryazan bomb squad Yuri Tkachenko who had performed the testing. Tkachenko has asserted that the gas vapor detector (which cost around 20,000$) functioned properly. The gas vapor detector is not washed, but is being checked on a schedule by a professional technician, because it contains a radioactive source.  Curiously enough, Yuri Tkachenko has only discussed technical characteristics of the gas analyzer, but didn’t say that the gas analyzer has been used to perform the testing. 
In March 2000, Yuri Maximov has revealed that the Ryazan bomb squad possesses two explosive detectors. One is a gas vapor detector “M-02”, which is not very accurate and sometimes malfunctions under bad weather conditions. The bomb experts don’t like it and prefer to use the different analytic tool called “Exprei” which is more accurate. The latter tool has shown the presence of RDX when the analysis was performed by Yuri Tkachenko. Which might be explained by the presence of RDX particles on his hands. The contamination could have been prevented if he wore single-use gloves, but there were no funds to buy them. 
Shortly after that, in March 2000, head of the Ryazan FSB branch Sergeyev has provided the following comments for the program “Independent investigation” broadcast on a national TV: “One had to perform the express analysis urgently. It was wet and dirty outside. They have cut the bags and taken the specimens out. They have taken a briefcase containing their equipment, put it on a floor and poured sugar on the lid of the briefcase. Then they have scrubbed the sugar with pieces of paper that are sensitive to vapors, and change the color when they are sprayed from a can. Like the litmus paper test from high school. They have seen the presence of RDX vapors, and initiated the case. After that we have figured out that they had taken that briefcase to all drills and all deminings, and I think they had taken it to Chechnya, too. The lid of the briefcase was contaminated with RDX vapors. And it — not the sugar, but the lid of the briefcase — has shown [the presence of RDX].” 
A year later, Yuri Tkachenko said during a press-conference that the gas vapor detector has not been used during the Ryazan incident. 
Finally, from the proceedings of a criminal case that were revealed to the press in 2003, we learn that the Ryazan FSB has provided the paperwork which stated that the express analysis was performed using the explosive vapor detector M-02. But Yuri Tkachenko has said during the questioning that he had used the device “Exprel”. The police investigators have resolved the contradiction in the following manner. The bomb squad is equipped with the gas vapor detector M-02, but the latter has its limitations, and doesn’t work properly in the wind and with the polluted air. So they have used the device “Exprel”, which is more accurate and easier to use. But since they were not supposed to have it, the police bomb squad has filed in the papers claiming that they have used the gas vapor detector M-02. 
5.2. The analysis
Whoa! That’s been quite a mess.
Definitely, one can see the common pattern among the cited sources: the initial media reports have been wrong. The gas vapor detector (“M-02”) was not used, and the tool that was used (“Exprei” or “Exprel”) — which is some sort of a chemical analysis kit — provided the false positive result due to the contamination.
Still, it doesn’t sound much convincing. That’s why Felshtinsky and Litvinenko believe that the initial media reports have been correct, and the gas vapor detector has shown the presence of RDX vapors during the express analysis. Which proves that the sacks in the basement contained RDX, and were an actual bomb.
According to Felshtinsky and Litvinenko, further media reports, like the interview with Yuri Maximov, were nothing else but an attempt to cover up the situation which they condemn in no uncertain terms: “We had no right to demand heroism from the investigator. Maximov had a family, just like the rest of us, and it would have been impractical and dangerous to oppose the leadership of the FSB.” 
Except for the nauseating moralism, is there anything wrong with their analysis? Essentially, Felshtinsky and Litvinenko have used the Occam’s razor: if there’s a contradiction, the simplest solution tends to be the right one.
To answer that question, let’s examine something that Felshtinsky and Litvinenko have mostly overlooked — the specs of the MO-2 (which is its correct name) gas analyzer. Actually, they tried to make sense of it: “According to the gas analyzer’s technical specifications, it was both highly reliable and highly accurate, so that if the results of an analysis indicated the presence of hexogene fumes in the contents of the sacks, there should be no room for doubt.” 
We concur with that statement.
However, has the gas analyzer been actually used? If bad weather makes it unreliable as the media reports suggested, that might be a sufficient reason for the police to use a different method to detect explosives. So far no evidence to back up that claim has emerged. Could we corroborate the reports of unsatisfactory performance of the gas detector under bad weather conditions?
Let’s look at the specs.
5.3. The MO-2 detector
The MO-2 detector is a portable, hand-held device that detects vapors of explosives using ion-mobility spectrometry (IMS).  According to the specifications, its detection limit of RDX vapors is 10 part per trillion (ppt, v/v) — which is quite decent for that type of a device. [24, 25]
Now, what are we going to do with that information?
The gas phase of RDX exists only below a certain pressure, which is referred to as the saturated vapor pressure and depends on the temperature. Quite naturally, the said circumstance limits the utility of explosive vapor detectors.
Figure 1 shows the dependence of the vapor pressure of RDX on the temperature — which in our case equals the ambient air temperature. We can see that the detection limit of 10 ppt (v/v) corresponds to 28 degrees Celsius.
According to the data provided by the All-Russia Research Institute of Hydrometeorological Information, on the day of September 22, 1999 in Ryazan, the mean air temperature has been 10.6 °C, with the maximum value of 14.9 °C and the minimum value of 7.8 °C. 
Even if we assume that during the time when express analysis was performed the air temperature was the hottest during the day (15 °C) — which is overly optimistic — the said temperature corresponds to the concentration of RDX vapors as low as 1.1 ppt (see Figure 1). That value is an order of magnitude less than the detection limit of the MO-2 detector. Which means that the gas analyzer could not be used to detect RDX vapors.
At this point a critic would say: “They just had to heat up the substance! That would solve the problem with the vapor pressure of RDX.” Indeed, modern IMS detectors (such as “MO-2M” , an updated version of the “MO-2” detector) are often equipped with a preheating chamber which allows analyzing samples of unknown substances. Was it possible for the police bomb squad in Ryazan to use some kind of an improvised preheating chamber? In principle, yes. However, that would create additional routes for contamination — in that respect, it has been asserted in the press that the gas analyzer is not being washed , which excludes the possibility of such a setup. Moreover, such a possibility has not been mentioned in the media, which makes it a mere speculation.
That proves that Felshtinsky and Litvinenko have been wrong in their analysis, and the initial media reports that an explosive vapor detector has detected the presence of RDX vapors have been false, just as further media reports asserted.
So, what sort of equipment has been used?
The device used to detect the explosives has been identified in the press as either “Exprei” or “Exprel”. [12, 21] There is no device with such a name. However, it’s exactly how Russians — especially those not particularly proficient in English — would spell “Expray”.
What is it?
5.4. The Expray kit
Expray is a colorimetric explosives detection test kit, suitable for field use. It is being extensively used in a range of military and civilian applications and is commercially available from multiple vendors. [29–31]
The kit — shown on Figure 2 — consists of three aerosol cans and dispensers for collection papers.
To perform the testing, a sticky paper is swept across the studied surface (or substance), after which the paper is sequentially sprayed with the contents of the aerosol cans. A change of the color (see Figure 3) means that the explosives have been detected. Each aerosol can is capable of detecting a specific class of explosives.
The Expray kit matches the description of the equipment which — as major general Sergeyev claimed in a TV program broadcast in March 2000 — has been used to perform the express testing in Ryazan.  The only discrepancy is that the kit doesn’t react to vapors of explosives — however, that’s an obvious mistake.
Still, Expray has the sensitivity of 20 nanograms [29,30], which makes it capable of detecting small particles of explosives that are not visible to a naked eye. It is indeed possible that small particles of RDX could have stuck either to the hands of Tkachenko or to the lid of the briefcase containing the Expray kit. It’s likely complicated to locate the exact source of contamination, which might explain some discrepancy in the media reports.
The gas analyzer has not been used to perform the express testing. The presence of RDX has been determined by analyzing a sample of the sugar-like substance using the Expray explosive detection test kit. It is quite possible that there has been a false detection, which might be explained by the contamination of either the lid of the kit or the hands of Yuri Tkachenko with small particles of RDX.
6. An eyewitness account of the substance’s appearance
The initial media reports of the Ryazan incident stated that the substance found in the sacks resembled sugar. [7,17]
However, after several months that information has been challenged by an eyewitness.
In March 2000, Alexei Kartofelnikov has participated in a televised debate about the Ryazan incident. In particular, he has shared that during the incident he had managed to observe the contents of the sacks from the distance of three meters. And the contents of the sacks had a yellowish color and resembled granules, like chopped vermicelli. 
After a few years, Kartofelnikov has told a Moscow Times reporter that the contents of the sacks looked yellow and did not look like sugar, but more like rice. 
Proponents of the conspiracy theory believe that what Kartofelnikov has seen was actually RDX.
But does his account necessarily contradict the earlier media reports?
Actually, Russia’s standard for white granulated sugar permits it to have a yellowish hue , which might explain the color observed by Kartofelnikov.
The more interesting point concerns the size of the particles of the substance.
Due to limitations of the human vision, most people are unable to resolve the millimeter marks at distances greater than three meters  — and that is the distance from which Kartofelnikov has observed the substance.
However, the typical grains of sugar have the size of 0.4–0.7 millimeters. That means that if Kartofelnikov has observed granulated sugar from the specified distance, he would be unable to resolve the individual grains, and consequently, won’t recognize that the substance is actually sugar.
An immediate implication is that if the substance contained small lumps of sugar, Kartofelnikov would perceive them to be monolithic solids.
The eyewitness account of the appearance of the substance, provided by Alexei Kartofelnikov, could correspond to white granulated sugar observed from the distance of three meters.
7. The peculiar case of private Pinyaev
In March 2000, Russian newspaper “Novaya Gazeta” has published a story about the incident which has allegedly taken place in the Autumn of 1999 in a military unit located nearby Ryazan.  Private Alexei Pinyaev was tasked with guarding a munitions warehouse. At one occasion he and his pal have sneaked inside the warehouse, where they have seen a large quantity of 50 kg sugar sacks. They have poured some of the sugar into a bag. However, when they have tried to make some tea with the sugar, they were surprised to find out that the tea has been revolting and not sweet. Alarmed, they have found their platoon commander, who has given that sugar to a sapper who in turn has identified it as RDX.
Later, the story in Novaya Gazeta was refuted by the officers from the military unit (including its commander), as well as by Alexei Pinyaev himself. [12, 20]
Proponents of the conspiracy theory believe that Alexei Pinyaev was tasked with guarding sacks with RDX that were used during the Ryazan incident.
Is it possible to independently assess the validity of the story in Novaya Gazeta? Let’s have a look!
First, the story claimed that the tea made with the “sugar” from the munitions warehouse had a revolting taste. The implication was that that’s how RDX tastes. But actually, RDX has no taste.  Usually, RDX is stored in a phlegmatized form — which would make it the Composition A, i.e. the RDX with a few percent of wax. However, waxes are typically tasteless and couldn’t account for the revolting taste, either. 
Second, private Pinyaev — and his pal — have reportedly used the substance from the sacks to make some tea. The story states that they have noticed something odd when they’ve tasted the tea. Actually, they should have seen something alarming sooner than that. Unlike sugar, RDX does not dissolve in water.  So, they should have noticed that the substance wouldn’t dissolve in the hot beverage, as expected from sugar. That has not been mentioned in the article — which makes it fail an important common sense check.
Third, the story states that Pinyaev and his pal have addressed the platoon commander because they were concerned about their health after tasting the tea. Apparently, nothing was made about it. The story is narrated in a light, almost humorous tone — as if RDX were a harmless chemical. Actually, ingesting small quantities of RDX can lead to an acute poisoning with distinct neurotoxic effects.  The median lethal dose of RDX is 200 mg/kg, which amounts to 15 grams for an averagely weighted person.  Even if the ingested dose was lower than that, private Pinyaev — and his pal — should have been immediately taken to a hospital and administered an antidote, or they could have entered a coma within 48 hours.
The story of private Pinyaev revealed by Novaya Gazeta in March 2000 either misstates the essential properties of RDX or makes wrong implications about them, which exposes the story as a likely fake.
8. Why did the substance fail to detonate?
According to the initial media reports, bomb disposal experts have taken a kilogram of the substance found in the basement of the apartment block in Ryazan to a test range nearby the city. They have tried to explode the sample of the substance, but the detonation hasn’t occurred. 
Several explanations of the test were provided in the press by various authors. On September 24, it was suggested that the results of the test meant that most likely the bomb in Ryazan wouldn’t have exploded, either.  Other publication on the same day has suggested that the detonation hasn’t occurred, because the bomb disposal experts have taken a sample from the upper part of the sack, while the major concentration of RDX might have actually been in the lower part of the sack.  On September 28, yet another author has suggested that the detonation hasn’t occurred because the quantity of the substance taken for the test was too small. 
The proponents of the conspiracy theory believe that the results of the test should be dismissed for either of the reasons mentioned in the press.
Is it really so? Was it sufficient to attempt exploding only a kilogram of the substance? What if the part of the sack which the bomb disposal experts have examined contained only sugar, while the RDX was placed in some other part of the sack?
Let’s try answering these questions.
8.1. The critical diameter of RDX
The ability of a substance to explode is usually characterized by its critical detonation diameter. If a charge of an explosive is shaped like a cylinder, the detonation would spread across its length only if the diameter of the cylinder is greater than (or equal to) the critical diameter. 
Naturally, the critical diameter limits the minimum amount of the explosive that can be used in a device. The critical diameter of RDX is less than a few millimeters for its conventional formulations, which allows to create gram-scale devices using that explosive. [44,45] So, if the substance found in Ryazan was RDX — as proponents of the conspiracy theory believe — a kilogram of the sample would be quite sufficient for a successful detonation at the test range.
In other words, the results of the test prove that the substance found in Ryazan was not RDX.
Now, if the Ryazan incident has been a genuine attempt by the FSB to explode an apartment block, and the FSB had access to such a good military explosive as RDX, they would have no incentive to use a worse explosive (that would be harder to explode and have a lower yield). Still, for the sake of completeness, we shall consider such a possibility.
8.2. The detonator size
What if the substance found in Ryazan was an imperfect, low-density explosive with a large critical diameter, that could only be exploded in a quantity exceeding one kilogram? To account for that possibility, let’s use some additional information.
The detonator found in one of the sacks was a 12-gauge shotgun shell.  According to media reports and to Yuri Tkachenko, the shell was filled with gunpowder [17, 20], which cannot initiate common formulations of secondary explosives.
However, proponents of the conspiracy theory are concerned that if the shell was loaded with a potent primary explosive, it could be used as a detonator. For the purposes of the present discussion, let’s assume that that has been the case.
What matters is that the inner diameter of the shell would limit the critical diameter of a substance that might be detonated. Indeed, standard №8 blasting caps have the inner diameter of 6.2 mm and cannot detonate granular explosives with the critical diameter in excess of 15–20 mm. [43,46] Invoking similarity, we can estimate that a 12-gauge shell, which has the inner diameter of 18.5 mm, could only detonate the explosive substance whose critical diameter was less than or equal to 45–60 mm.
How much is 60 mm? If one kilogram sample of the sugar-like substance were shaped like a cylinder with that diameter, its length would be around 400 mm — that kind of an aspect ratio implies that the amount of the substance was quite sufficient for a test. 
According to the media, the bomb disposal experts have attempted to blow up the sample of the substance using a detonator based on a 12-gauge shell.  Apparently, the experts knew their job. Trying to explode a sample larger than 1 kilogram was unnecessary, because that amount is quite sufficient for any explosive that could be detonated using such a setup.
Now we have reached the answer to one of our questions. If the substance used in the Ryazan incident was an explosive with a large critical diameter — that required a sample larger than 1 kg to perform the test — that also means that the substance couldn’t have been detonated with a modified 12-gauge shell.
8.3. The shock wave
But we have actually one question left. What if the part of the sack containing the detonator was filled with sugar, while RDX was located in the bottom part of the sack? Could it have fooled the bomb disposal experts?
In such a setup, there would be no continuous path for the detonation to spread from the modified shell to the part of the sack containing RDX.
But is that necessary? Could the RDX in the bottom of the sack have been initiated by the shock wave from the detonator?
Apparently, it couldn’t. The detonation pressure of common types of explosives is about 10–15 times more than the critical pressure that could initiate the detonation.  Upon leaving the detonator, the energy per unit area of the shock wave would decrease following the inverse square law with the distance from the detonator. That means that a gap of 6–8 centimeters between the detonator and the RDX would be quite sufficient to prevent the detonation by a shock wave.
On September 22, 1999, a suspicious device was found in the basement of the apartment block in Ryazan (14/16 Novoselov Street). It contained three sacks filled with a sugar-like substance and a 12-gauge shell used as a detonator. The bomb disposal experts have tried to explode one kilogram of the substance, using the similar kind of a detonator at the test range. The substance has not detonated. That means that the device found in the basement has actually been a dummy bomb.
In this article we have shown that the Ryazan incident of September 1999 has actually been a training exercise by the Russian security services.
The conspiratorial account of the Ryazan incident — that it has been a deliberate attempt of the Russian security services to blow up a block of apartments — is a fine example of pseudoscience. It appears to be logical, however a careful examination of its claims proves it wrong.
Contrary to that, the official version of the Ryazan incident — that it has been a training exercise — is an example of bad communication. The way it has been presented in the media makes it look rough at the edges. However, when we scrutinize its claims, we see that they are actually grounded in the reality.
Now we can answer our initial question: is there any proof of the theory which blames the Russian security services for the apartment bombings? We have shown that there is no evidence to support this conspiracy theory.
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