- Dzhokhar Tsarnaev told investigators "no other attacks" were planned
- He told FBI he called his brother to "try to synchronize the two detonations"
The records have been unsealed as part of the appeal of Tsarneav's friend, Robel Phillipos,
who was sentenced last June to three years in prison for making false statements to law enforcement in a terrorism investigation.
Interviewed by FBI agents at a Boston Hospital after his arrest, Tsarnaev also told investigators "there were no other attacks planned" and only he and older brother Tamerlan -- who was killed in a chase and gunbattle with police days after the bombings -- were involved in the attacks' "planning and execution," according to the records.
A federal jury sentenced Dzhokhar Tsarnaev to death in June.
The six counts that brought Tsarnaev a death sentence all related to the second of two pressure-cooker bombs, which caused the explosion on Boylston Street in front of the Forum restaurant on April 15, 2013. He was not sentenced to death for the first bomb, which was planted by Tamerlan, nor for the shooting death of MIT officer Sean Collier.
The younger Tsarnaev told agents that "no one had any knowledge of their plan before the attack, nor did they tell anyone after the attack had occurred," the documents said. They kept their plan a secret "because they could not trust anyone else."
Tsarnaev did not warn friends to stay away from the marathon because "he didn't care if they got hurt," according to the records.
The bombs were constructed in Tamarlan's home with instructions from Inspire magazine, an online publication produced by al Qaeda's Yemeni affiliate, al Qaeda in the Arabian Peninsula, Tsarnaev told investigators, according to the documents.
The powder for the explosives came from $200 worth of fireworks bought in New Hampshire. He said Tamarlan's home offered more privacy to make the explosive devices because Dzhokhar Tsarnaev had a roommate.