BOSTON — A close friend of Boston Marathon bomber Dzhokhar Tsarnaev told a jury that Tsarnaev borrowed a gun from him two months before the 2013 attack and the killing days later of a police officer.

Prosecutors have identified the Ruger handgun as the weapon used to kill Massachusetts Institute of Technology Police Officer Sean Collier.

Stephen Silva testified Tuesday at Tsarnaev’s federal death penalty trial that after Tsarnaev borrowed the gun, he repeatedly asked him to return it, but Tsarnaev “kept coming up with excuses” for not bringing it back.

Silva, 21, described Tsarnaev as a friend he had known since the eighth grade. He has a plea agreement with prosecutors that he hopes will help him get a reduced sentence on drug and gun charges.

Three people were killed and more than 260 were wounded when two bombs exploded near the marathon finish line on April 15, 2013.

During opening statements, Tsarnaev’s lawyer admitted he participated in the bombings, but said his older brother, Tamerlan, was the mastermind. His lawyer said it was Tamerlan who used the gun to shoot Collier.

During cross-examination of Silva, Tsarnaev’s lawyer suggested again that Tamerlan was a domineering older brother who had a powerful influence over Dzhokhar. Attorney Miriam Conrad asked Silva if he knew why Dzhokhar had never introduced him to his brother.

“Yes,” replied Silva. “He said his brother was very strict … very opinionated, and that since I wasn’t a Muslim, you know, he might give me a little (expletive) for that.”

Tamerlan Tsarnaev died following a shootout with police. Dzhokhar, who was making his escape, ran over his brother as three police officers were trying to handcuff Tamerlan on the ground.

Dzhokhar Tsarnaev was found hours later, hiding in boat parked in a Watertown backyard. On Tuesday, boat owner David Henneberry described how he discovered a wounded and bloodied Tsarnaev.

Henneberry said that on April 19, 2013, he looked outside and noticed that the shrink wrap on his boat looked loose. When he looked inside, he noticed blood.

“I just kept fixating on this blood … my eyes looked on the other side of the boat and that’s when I saw a body on the boat,” Henneberry said. It was Tsarnaev, lying on his side.