Boston Marathon bomber Dzhokhar Tsarnaev, in interviews from his hospital bed, told FBI agents that he and his brother acted alone in building the explosives and planning the attack that killed three people, according to newly released court documents.

Tsarnaev communicated with investigators before receiving his Miranda warning in the days after the bombing on April 15, 2013, the court papers say. By then, his older brother, Tamerlan, was dead after a shootout with police, and Dzhokhar was suffering from gunshot wounds to his face and jaw sustained when cops closed in on him.

Unable to speak, Dzhokhar answered the FBI agents' questions in handwritten notes. Writing in thick black ink on notebook paper, he told the investigators that he and his brother planned the design of the bombs, procured bomb parts and built the bombs that also injured some 260 people near the marathon's finish line, according to FBI reports known as 302s that were submitted to federal court as part of a 312-page filing.

Dzhokhar said the bombs were built at his brother's apartment in Cambridge, Massachusetts, the documents say.

Dzhokhar Tsarnaev. FBI

The statements provide answers to some of the lingering questions surrounding the attack: Where were the bombs built? And did someone else provide them to the brothers?

FBI agents testified at Dzhokhar's trial that they could not find traces of explosives at the apartment, even after pulling drains in the hope of finding explosive residue.

In his statements to the FBI, Dzhokhar described to FBI agents how he and Tamerlan spent weeks crushing fireworks to amass enough black powder to build the bombs. The effort took place mostly in their home but sometimes in a van or in Dzhokhar's car, according to the documents.

Once the bombs were prepared, Dzhokhar told the investigators, the brothers began scoping out locations to set off the bombs on Friday, April 12, court papers say.

While they were surveying possible targets, including police stations in Cambridge and Boston, the two learned that the marathon was scheduled for the upcoming Monday, April 15. On Saturday, Tamerlan determined that they would target the race, Dzhokhar told the agents, according to the court papers.

The brothers finished the bombs on the day of the race after working on them for roughly two hours, according to an FBI document.

Dzhokhar told the agents that they had worked on the explosives during the day when Tamerlan's wife, Katherine Russell, was at work so she wouldn't find out about their plans.

Russell was investigated by the FBI about what she may have known about the plot and about web searches from her laptop, which included the phrases "wife of Mujahideen," or Holy Warrior, and "rewards for wife of Mujahideen." But she was never charged by the U.S. attorney's office.

The statements also confirm what NBC News has previously reported, that the brothers originally intended to attack a Fourth of July event in Boston; that they attacked earlier because the bombs were ready sooner than they anticipated; and that two brothers planned to travel to New York and bomb Times Square on April 18 because the two had seen their pictures on the news as a result of an FBI news conference that afternoon.

However, Dzhokhar wasn't truthful in all of his statements to agents. For instance, he initially told them that he didn't know where his brother got the gun used to kill MIT officer Sean Collier but later said they found it in a junkyard.

An emergency responder and volunteers, including Carlos Arredondo, center in cowboy hat, push Jeff Bauman in a wheelchair after he was injured in one of two explosions near the finish line of the Boston Marathon on April 15, 2013 Charles Krupa / AP file

The gun was actually given to Dzhokhar by a friend, according to the testimony of the friend, Stephen Silva, and a video recording of Silva telling an FBI informant that he gave Dzhokhar the gun.

The handwritten notes indicate that Dzhokhar asked FBI agents several times whether Tamerlan was dead or alive. Tamerlan had been dead for nearly 48 hours at the time, according to public statements. However, the notes and the defense filing seems to suggest that the FBI had originally told Dzhokhar that his brother was still alive.

The statements were originally made in a sealed motion filed by Dzhokhar's defense attorneys who were arguing to the court that the statements should be excluded from being made public to the jury. The statements were not made public at trial but much of what Dzhokhar said came out in other testimony or in evidence presented by law enforcement.

On Wednesday, federal prosecutors asked a federal judge to put the documents under seal again as they were inadvertently released by the court.

Dzhokhar is being held at the most secure prison in the U.S., ADMAX Florence in Colorado. He is facing the death penalty but is appealing his case.