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Affidavit describes Dzhokhar Tsarnaev’s actions

When he arrived on Boylston Street on April 15 allegedly intent on using weapons of mass destruction against the unknowing fans of the Boston Marathon, Dzhokhar Tsarnaev wore a white baseball cap and a black jacket.

The FBI said today in an affidavit they found a white baseball cap and a black jacket when they searched Tsarnaev’s dorm room at UMass Dartmouth, two more pieces of evidence allegedly tying the college sophomore to the terrorist bombings.

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In an affidavit filed in US District Court today, FBI Special Agent Daniel R. Genck summarized some of the evidence collected by law enforcement since two explosions detonated last Monday at 2:50 p.m., killing three people and wounded more than 170.

The affidavit said that based on reviews of surveillance video, photos and videos, the man now called Bomber Two by federal law enforcement stood for four minutes next to the bomb he had placed on the ground in front of Forum Restaurant on Boylston Street.

He had arrived on Boylston Street carrying a backpack and accompanied by his older brother Tamerlan, whom the FBI now calls Bomber One.

Dzhokhar Tsarnaev had a cell phone in his hand and appeared to take a photograph of the bomb on the sidewalk before he walked away.

“Approximately 30 seconds before the first explosion, he lifts his phone to his ear as if he is speaking on his cell phone, and keeps it there for approximately 18 seconds. A few seconds after he finishes the call, the large crowd of people around him can be seen reacting to the first explosion,’’ Genck wrote.

“Virtually every head turns to the east (towards the finish line) and stares in that direction in apparent bewilderment and alarm. Bomber Two, virtually alone among the individuals in front of the restaurant, appears calm. He glances to the east and then calmly but rapidly begins moving west, away from the direction of the finish line.’’

Genck added: “He walks away without his knapsack, having left it on the ground where he had been standing. Approximately, 10 second later, an explosion occurs in the location where Bomber Two had placed his knapsack.’’

The affidavit also provided a detailed account of the events Thursday night and early Friday that led to the death of one suspect and the capture of the other.

According to the affidavit, about seven hours after the FBI released photos and videos of the brothers, then known only as Suspect 1 and Suspect 2, last Thursday, a person carjacked an SUV at gunpoint in Cambridge. The victim, whose name was not released, told police that the carjacker knocked on the window of his car.

When the victim rolled down the window, the armed man told him “Did you hear about the Boston explosion? ... I did that … I am serious,’’ according to the affidavit. The man removed the magazine from his pistol, showed the victim there was at least one bullet inside, and then reloaded the firearm, according to the affidavit.

The armed man – it’s not clear which brother was the armed man – got into the car and forced the victim to drive to another location where the victim was forced to get into the right front passenger seat while a second man climbed in the car and got into a rear seat. The men then talked to each other “in a foreign language.’’

The men put something in the trunk of the car, robbed the man of $45 in cash and ordered him to hand over his ATM card and password for the account. When they tried to get cash from his account, however, they did not succeed. The two men then drove to 816 Memorial Drive in Cambridge, where the victim escaped from the vehicle.

After the car was reported stolen, police spotted the vehicle on Dexter Street in Watertown and began a pursuit.

“As the men drove down Dexter Street in Watertown, they threw at least two small improvised explosive devices (IEDs) out of the car,’’ which was followed by a massive gunfight that ended with Tamerlan fatally wounded. Dzhokhar drove a short distance, abandoned the car and was later tracked down in Watertown on Friday night after a massive search that shut down large parts of greater Boston for the entire day.

Inside the abandoned car, an “intact low-grade explosive device was discovered.’’ From the shooting scene, the FBI also recovered “two unexploded IEDS, as well as the remnants of numerous exploded IEDs.’’

The affidavit also provided details of the bombs used at the marathon. The FBI described them as “low-grade explosives that were housed in pressure cookers. Both pressure cookers were of the same brand. The pressure cookers carried metallic BBs and nails. Many of the BBs were contained within an adhesive material. The explosives contained green-colored hobby fuse.’’

The IEDs recovered after the gunfight were made using the same brand of pressure cooker and also had “metallic BBs contained within an adhesive material as well as green-colored hobby fuse. The intact low-grade explosive found in the abandoned car was in a plastic container and wrapped with green-colored hobby fuse.’’

Inside Tsarnaev’s room at the Pine Dale Hall, the FBI also found a “large pyrotechnic’’ and BBs.

John R. Ellement can be reached at ellement@globe.com. Follow him on Twitter @JREbosglobe.

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