Newly unsealed documents reveal that Boston Marathon bomber Dzhokhar Tsarnaev told FBI agents that he and his brother picked their own spots to place their bombs and that each of them set off his own device.
Tsarnaev talked to FBI agents at a hospital where was undergoing treatment for gunshot wounds. The interviewed him six days after the 2013 bombing, but a summary of his statements, prepared by the FBI, had not previously been disclosed.
According to the FBI summary, Tsarnaev told the agents he and his older brother, Tamerlan, walked toward the marathon finish line on the day of the bombing, April 15, each carrying a backpack containing a powerful bomb. He said he called his brother just before the attack to "synchronize the two detonations." Tamerlan set off the first bomb, and Dzhokhar's followed ten seconds later.
"After JAHAR put his backpack down, he walked away and detonated the device using a button," the FBI document said. The two blasts killed three people and severely wounded more than 200 others.
FBI agents testified at Tsarnaev's trial that investigators recovered the detonator used to set off the bomb placed by Tamerlan, but the second detonator was never found. That left a question unanswered: did Tamerlan detonate both, or did each of them detonate the bombs they placed?
The bombs were built, he said, in Tamerlan's apartment using instructions downloaded from al Qaeda's online magazine, Inspire.
The FBI summary also said Tsarnaev told the agents that "there were no other attacks planned, there were no unaccounted devices, and the only individuals involved in the attack planning and execution were Jahar and Tamerlan," using the younger Tsarnaev's nickname.
The FBI document said Tsarnaev said no one else knew of their plans.
"JAHAR stated no one had any knowledge of their plan before the attack, nor did they tell anyone after the attack had occurred."
As for why did the younger Tsarnaev did not warn any of his friends to stay away from the marathon that day, "he didn't care if they got hurt," the FBI interview summary said.
This article originally appeared on NBCNews.com.