Boston bomber 'got gun from school friend'

A friend of accused Boston Marathon bomber Dzhokhar Tsarnaev has told a court he supplied the gun used in a murder during the post-bombing police chase.

A friend supplied the gun Dzhokhar Tsarnaev used during a post-Boston Marathon bombing chase. (AAP)

A high school friend of accused Boston Marathon bomber Dzhokhar Tsarnaev supplied the gun used in a murder, carjacking and violent police confrontation during the post-bombing police chase, the friend has told a federal court.

Stephen Silva, 21, whose drug dealing led to his later arrest and revelation about the gun, told the court that he and Tsarnaev had been "very, very close."

The two teenagers went to separate universities after high school, but whenever they met, they would hang out like old times, smoking marijuana, talking, drinking, "typical teenage stuff," Silva said on Tuesday.

Silva had been keeping the P-95 Ruger pistol since 2012 for another friend, and during one of their encounters, Tsarnaev "asked me to potentially borrow the gun," Silva said. Tsarnaev said he wanted to "rip," or rob, some students.

He passed it on to Tsarnaev in February 2013. Later that month, Silva tried to get it back, but Tsarnaev "just kept coming up with excuses," Silva testified.

Again in April, shortly before the April 15 twin bombing that killed three and injured more than 260, Silva tried to get the gun back, but Tsarnaev just brushed aside the question. Tsarnaev had come to buy marijuana from Silva, and before they parted, Silva told Tsarnaev he loved him, Silva testified.

After learning that Tsarnaev and his brother Tamerlan, who died in the police chase, had bombed the marathon finish line, "I was in a state of huge shock, disbelief, paranoia," Silva testified.

He ditched his phone and temporarily stopped selling drugs, which had been earning him up to $US2,000 ($A2,625) a week. But then he unsuspectingly sold heroin to an undercover informant, casually telling him that he was the one who gave a gun to a Marathon bomber.

Silva pleaded guilty in December 2014 to gun and heroin charges. He faces 40 years in prison, and told the jury that he is testifying in hopes of a lighter sentence.

Tsarnaev's defence lawyers have not disputed that Dzhokhar planted one of the two bombs. But they are trying to spare the 21-year-old from the federal death penalty, arguing that older brother Tamerlan, who practised radical militant Islamism, had unduly influenced their client.