The bombs sounded like fireworks. The screams sounded like the cheers that had poured through my open window for hours. It wasn’t until I saw “explosion at the finish line” on Twitter that I took out my camera bag, snapped on my zoom lens and ran out the door, shoes untied and sockless.

A mother was comforting her crying child – no older than the boy who I would later learn was killed minutes earlier. The crowd from the marathon was streaming past me, north toward the river.

From across the street, it still looked much like the marathon always has: Police in bright green vests managed crowd control as spectators milled around, waiting for the runners. But as I approached the metal barriers on Commonwealth Avenue and Hereford Street, three blocks from the finish line, I could see it was all wrong.

A runner wept in the arms of a man who led her back up the street she’d just passed through, exhausted.

Another runner stumbled, supporting herself on the barrier. She hadn’t finished. Her name was Kit, and she asked me to send a text message to her family in Florida.

“Did she fall?” they replied. The news was still young.

Those were the only runners. It was just after 2:00 on the afternoon of Marathon Monday, Boston’s moment in the spotlight, and the flow of delirious athletes had stopped. Only men in uniform were running to the finish.

The ambulances started arriving before the crowd dispersed. The blaring siren joined the others echoing down Hereford St. Police tried to clear a path through the chaos.

The crowd thinned, and Boylston Street cleared. The police presence had thickened and a few members of the National Guard were arriving, as if out of nowhere.

Spectators left at the urging of police and a growing number of Guard troops. Those who remained frantically checked their cell phones, dialing and redialing to try to reach their runners on the overburdened cell networks. Despite rumors of cell blackouts in the afternoon, I was able to send text messages and tweets all day.

A few lucky families connected with their loved ones. Others remained panicked, holding out hope, for the first time all day, that theirs had been the slow ones, stopped on Commonwealth Avenue, about a mile short of their goal.

“The runners are starting to suffer from hypothermia,” a Boston police officer said. “We need to get them out of here.” They tried to coordinate busses to bring people across the Charles to MIT, but a crowd that large – even one made up of runners – doesn’t move quickly. Family members made their way toward the stagnant mass of runners, holding their last-mile signs high for all the wrong reasons.

Eventually, the runners were allowed to proceed, but instead of turning right on Hereford and left on Boylston, down the final stretch among cheering fans, they’d turn left, away from the finish. Some jogged anyway, determined to run 26.2, whether it counted or not. The onlookers that remained applauded.

A couple found each other then. They shared a long hug, then he shared a sweater.

Many other runners passed wearing only trash bags to cut the cold.

Half a dozen more ambulances filed in, then the trucks. Bigger, blacker trucks rolled into the city all day.

Some looked bulletproof, some I’d never seen the likes of before. They were all filled with people wearing dark clothes and guns.

The runners, an officer told me, had been moved to the Common, where they would gather their things and find a place to stay for the night. But there were a lonely few of them there, and they were far outnumbered by men with guns.

A young father, the baby strapped to his chest nearly tangling in police line, had a question, and a SWAT officer who looked more fit for a war than a Boston sidewalk on a Monday afternoon, casually answered. “They haven’t told us anything,” was a common response on the Common, even when I asked where the runners were.

Across the field a person sat awkwardly, hands bound, in the grass. Two officers stood casually nearby. I took to Twitter with a photo of the scene. In the pixelated sunlight, I could make out a white hooded sweatshirt, the hood down, and the bright white orb of the detained person’s headwear. It was impossible, from such a photo, to draw any conclusions.

Tweets came back fast. “What is the guy wearing that they are standing around?” and “Maybe it's just me, but is the individual wearing a headscarf?”