Two young men with backpacks walked with purpose down Boylston Street Monday afternoon, weaving through the crowd on the sidelines of the Boston Marathon. It seemed like they'd been there before, like they knew where they were going.
The one in the white cap reached his destination first, about two blocks from the finish line. The other one, wearing a hoodie and a black cap, kept going. Some three minutes later, he elbowed his way through the crowd and dropped his backpack near the finish line. It was about 15 minutes before 3 p.m.
The first explosion, at 2:50 p.m., sent smoke and flames into the air -- and glass and nails and ball bearings and BBs into the crowd. It seemed to inflict the cruelest kind of damage to any marathon fan: It attacked their legs.
Jeff Bauman, who survived but lost both legs, saw the man in the black cap drop his bag. Two women standing nearby -- restaurant manager Krystle Campbell and Chinese grad student Lingzi Lu -- died in the blast.
As some people fell and others ran screaming, the man in the black cap casually walked away.
Twelve seconds later, another explosion, more screams, more panicked people running. This time, a little boy, Martin Richards, 8, lay on the sidewalk, fatally injured. His mother and sister also were seriously hurt. In the crowd, the man in the white cap strolled calmly and turned the corner onto Fairfield Street.
Later, an official who asked not to be named told CNN: "When the bombs blow up, when most people are running away and victims were lying on the ground, the two suspects walk away pretty casually.
"They acted differently than everyone else."
That night, a few minutes after 8, a college student using the screen name J_tsar tweeted a quote from rapper Jay-Z:
"Ain't no love in the heart of the city. stay safe people."
For the next three days, Boston and the rest of the nation wondered who was behind the first terror attack of its kind on U.S. soil since September 11, 2001. Was it al Qaeda, a homegrown terror group or a lone wolf?
Because the bombing was suspected terrorism, the FBI quickly took the investigative lead. But there was no chatter among the jihadists. No one claimed responsibility for the blast.
But J_tsar was a chatterbox on Twitter. Early Tuesday, just past midnight, he sent out another, more mysterious tweet: "There are people that know the truth but stay silent & there are people that speak the truth but we don't hear them cuz they're the minority."
All day Tuesday, the news focused on the gruesome details of the crime scene and those who were lost or maimed. More than 170 people were hurt by flying glass, shrapnel, ball bearings and nails, some of them grievously. The sidewalks along Boylston Street were slick with blood in some spots.
President Obama condemned the marathon bombings as terrorism and vowed that those responsible would feel the full weight of justice.
Sometime that day, a college student named Dzhokhar Tsarnaev came into Gilberto Junior's body shop in the suburb of Somerville. Junior said he had known Dzhokhar for about two years and that he had a taste for expensive cars and clothes. Dzhokhar had dropped the car off two weeks earlier, said it was his girlfriend's and then "disappeared."
Although Junior was still working on the car and it was missing a rear bumper and tail lights, Dzhokhar said he needed his white Mercedes station wagon immediately. He appeared nervous, Junior said, and was biting his nails. He wondered if his customer was under the influence of drugs.
That night J_tsar tweeted up a storm:
He quoted the rapper Eminem: "Nowadays everybody wanna talk like they got somethin to say but nothin comes out when they move their lips; just a bunch of gibberish."
He slapped down an Internet rumor that a man had planned to propose to his girlfriend at the marathon but found her dead: "fake story."
He replied to someone else's tweet: "and they what 'god hates dead people?' Or victims of tragedies? Lol those people are cooked."
And he tweeted: "So then I says to him, I says, relax bro my beard is not loaded."
Behind the scenes, federal investigators began to sort through what has become the norm in a post-9/11 society: Thousands upon thousands of surveillance photos and videos taken from cameras at traffic lights, store fronts, parking garages and other places along the marathon route.
The crime scene extended for 12 blocks. The 26.2-mile marathon route is open to the public and the event is heavily photographed. Authorities asked for amateur cell phone photos and videos from anyone who had been at the marathon. Who might investigators find on the sidelines, in the background?
During a shift change at the Boston Police Department, a supervisor told officers: "When you get home tonight hug your kids once and then hug them again. And that's an order."