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."
As Tuesday melted into Wednesday, J_tsar was back on Twitter. "I'm a stress free kind of guy," he tweeted shortly before 2 a.m.
The day seemed like any other at school for the Mercedes-driving 19-year-old later identified as the tweeter: Dzhokhar Tsarnaev, a sophomore at the University of Massachusetts-Dartmouth. His student ID told the story of his day; like all students, he has to swipe the card to enter buildings on campus.
Card swipe information shows he went to the gym and spent Wednesday night at his dorm. Dzhokhar was known as Jahar to friends on campus.
Student Zach Bettencourt said he discussed the bombing with Dzhokhar at the gym.
"You hear about this kind of thing happening in Iraq and Afghanistan but not here," Bettencourt said.
Dzhokhar responded: "Yeah tragedies happen man, like these things happen around the world. It's crazy."
Less than 48 hours after the bombing, Harry Danso was making small talk with Dzhokhar at their dorm. "He was just in the hallway, said 'Hi' and walked past me. He just acted regular. Gave me a regular smile, like usual."
He also went to a party at the dorm, a fellow student told The Boston Globe. It was attended by friends who competed in intramural soccer.
"He was just relaxed," the student said, asking the paper not to publish her name. Also on Wednesday, authorities revealed that one and possibly both of Monday's deadly devices had been fashioned out of pressure cookers. A pressure cooker lid was found on a rooftop near the marathon finish line.
Meanwhile, Dzhokhar's older brother, Tamerlan, was reaching out to family members. He called two uncles on Thursday, seeking their forgiveness.
"He called me, confused," Ruslan Tsarni, who lives outside Washington, told CNN. In an earlier interview with USA Today, another uncle quoted Tamerlan as saying, " 'I love you and forgive me' ... I guess he knew what he had done."
More violence, and a breakthrough
Thursday was a breakthrough day for investigators.
They already had made progress, finding clear images of the men with the backpacks and ball caps on a surveillance video. Intelligence had been developed on one suspect earlier in the week; images of the second suspect were isolated Wednesday, officials told CNN.
Jeff Bauman, who'd survived the bombing but lost both legs, regained consciousness at Boston Medical Center and gave them a lead. On a piece of paper, he wrote: "Bag, saw the guy, looked right at me."