BOSTON (CNN) -- When the first bomb went off everybody turned their heads -- except for one person. The young man with the turned-around white ball cap had been standing by a tree, and he casually strolled away, leaving his backpack behind.
He was looking over his shoulder when the second explosion came.
Federal prosecutors on Monday showed a video compilation of images, some never before seen by the public. They were taken from surveillance cameras along the route leading to the finish line of the Boston Marathon on April 15, 2013, the day three people were killed and more than 240 people were injured by hot shrapnel, nails and BBs from two pressure cooker bombs that had been hidden in backpacks and dropped amid the crowd.
The man prosecutors say is defendant Dzhokhar Tsarnaev strolls unnoticed through the crowd. He stops by a tree and looks to his right, and then to his left. He drops a heavy backpack to his feet, a few feet behind a line of children. He makes a phone call.
Federal prosecutors say that 19-second call was to his brother, Tamerlan Tsarnaev, who also is shown on the video. They arrive together, turning the corner from Gloucester Street onto Boylston Street. Then they split up.
"We called them Black Hat and White Hat," testified Anthony Imel, a digital forensics analyst for the FBI. Prosecutors say Black Hat is Tamerlan Tsarnaev, at 26 the older of the pair. He died in a gunbattle with police a few days after the bombings.
"White Hat," they say, is the defendant, Dzhokar "Jahar" Tsarnaev. At the time, he was 19. Now he is on trial for his life; 17 of the 30 counts against him carry a possible death sentence.
Tsarnaev has acknowledged that he and Tamerlan were responsible for setting off the bombs near the marathon's finish line.
By taking responsibility for the carnage, Tsarnaev is hoping the jury will spare his life after hearing the full story.
Federal prosecutors say it was the worst act of terror on U.S. soil since 9/11. Jurors were told 118 people were carted off in 90 ambulances within 20 minutes of the blasts.
On the third day of what had suddenly become a fast-paced trial, prosecutors began to build their case in earnest. Jurors heard more heart-wrenching testimony from the victims of the bombs. They watched videos of the brothers planting their backpack pressure cooker bombs and viewed an animation showing the path they took. They watched more videos showing Tsarnaev paying $3.49 for a half-gallon of milk at a Whole Foods store in Cambridge less than a half-hour after the bombings. And they saw him enter the gym on the campus of UMass Dartmouth shortly after 9 p.m. on April 16, the day after the bombings.
Surveillance video revealed
The video was put together from surveillance video collected by FBI agents in the aftermath of the bombing.
It shows Tamerlan Tsarnaev walking past the Forum restaurant on his way to the finish line, where he dropped his homemade bomb, fashioned from a pressure cooker packed with gunpowder, nails and BBs.
He is followed by Dzhokar, who lingers by a tree in front of Forum for about four minutes.
There is an explosion and everybody's head turns toward the finish line -- except Dzhokar Tsarnaev's. He casually walks away from his spot by the tree, without his backpack. He is looking back over his shoulder when the second bomb goes off 12 seconds after the first. It is not yet 3 p.m.
Then he can be seen running with the crowd fleeing the bombs he and his brother set off.
Prosecutors: Tweets show plot was on Tsarnaev's mind for months
On Monday, prosecutors also presented evidence they said showed targeting the marathon had been on Tsarnaev's mind for at least a year.
Jurors viewed posts from two Twitter accounts that FBI forensics analysts linked to Tsarnaev.
The first, @J_tsar, is fairly well known.
"I shall die young," @J_tsar tweeted on April 12, 2012.
"They will spend their money & they will regret it & they will be defeated," he tweeted on April 16, the day of the 2012 Boston Marathon.
Prosecutors also revealed a second account, which they say is evidence Tsarnaev led a double life. By day he was a slacker college sophomore who played video games and worked out at the gym with his friends. But late at night, he was a wannabe jihadist, posting on a second account, @Al_firdausiA.
The name evokes the highest level of paradise a martyr can achieve. He called himself Ghuraba, the stranger. In one tweet, he urged people to listen to radical cleric Anwar al-Awlaki's lectures.
"You will gain an unbelievable amount of knowledge," he said. Prosecutors also allege in the indictment that Tsarnaev downloaded al-Awlaki's writings, calling him a "well-known al Qaeda propagandist." He was killed in a U.S. drone strike in 2011.
Tsarnaev's other account, which was more mainstream, featured 1,100 tweets. Among them, the infamous note at 8 p.m. on the day of the bombing: "Ain't no love in the heart of the city, stay safe people." His final tweet, on April 16, the day after the bombing, was, "I'm a stress free kind of guy."
'There was smoke, there was blood'
Jurors also heard more wrenching stories of bombing victims at what has become an unsually fast-paced trial. Twenty-six people have testified in three days, including bombing victims, first responders and FBI agents and analysts.
Monday began with dramatic testimony from a nurse and newlywed who lost both her legs; her husband also lost a leg.
Jessica Kensky told the jury she didn't see or hear the explosion but felt as if she'd been launched on a rocket ship.
"I remember being happy, I remember feeling sunlight on my face. I remember feeling free," she said. And then came the explosion.
"I just felt like I was on a rocket," she said. "There was smoke, there was blood. I went into nurse mode. I was focused on my husband. His foot and part of his leg was completely detached, hanging on by a thread."
She used her purse strap to make a tourniquet to stop the bleeding from her husband's leg.
Then a man came to her and said, "Ma'am, you're on fire, you're on fire," she recounted.
She lost one leg almost immediately. She made a difficult decision to have the second amputated in January.
"I wanted to paint my toenails and put my feet in the sand. I wanted all of those things, and to lose my second leg was a gut-wrenching decision," she said.
Danling Zhou, a friend of bombing victim Lingzi Lu, testified how the bomb tore a gaping wound in her abdomen, and that she pushed on the wound "with all the strength I had" to keep her internal organs from spilling out.
She looked at Lu and thought she would be fine. The bomb blew out her eardrums and she couldn't hear her friend screaming but she could see her. Then Lu lapsed into unconsciousness with her hands over her face.
At the hospital, she asked about Lu every time she woke up. Finally, a friend told her that Lingzi was dead.
"I guess everyone found out earlier," she said. "They were trying to protect me, so they didn't tell me."
A physician who happened to be watching the marathon tried to help Lu, whose leg had been slashed from her hip to her ankle. When he heard what he called "agonal breathing," literally her last gasps, he said he knew he couldn't save her.
In her purse, he said, he found a piece of metal from the bomb. It was still warm.
CNN's Michael Pearson contributed to this report.
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