A childhood friend of Dzhokhar Tsarnaev apologized Tuesday for lending the Boston Marathon bomber the illegal gun that was later used to kill an MIT police officer, and in a firefight with police in Watertown.

Stephen Silva was never charged in relation to the 2013 Marathon bombings, but he confessed to living a life of drug dealing and gun trading, a life he now regrets. He was set free after being sentenced Tuesday to time served, or 17 months in federal prison.

“I had no idea that the firearm I lent to Tsarnaev would be used in the way it was,’’ he told US District Judge Mark L. Wolf. He wore tan prison garb, and kept his hands folded in front of him.

When confronted by the judge with the reality that he had provided the gun that was later used to kill MIT police officer Sean Collier, Silva bent over and said softly into a microphone, “I’m sorry for that.’’

“I wish I could go back in time and change my actions,’’ he told the judge, reading hastily from a piece of paper. “I was young, dumb, and thought I could outsmart everyone.

“Your Honor, I’m just pleading for a second chance.’’

Wolf, a veteran judge of 30 years, said the 22-year-old Silva was once a promising student who “went the wrong way,’’ and yet he recognized Silva’s determination to cooperate with authorities and testify against an old friend — testimony that prosecutors later said was material in the case against Tsarnaev.

“The fact that you did cooperate right away . . . does communicate to me that you intend to make your future different from your past,’’ the judge said.

The judge also acknowledged that Silva was willing to cooperate in the high-stakes trial, a decision that will follow him for the rest of his life, but added, “You were prepared to take the risk, and that reflects a lot, as well.’’

Silva was freed from the federal courthouse in South Boston less than three hours after the hearing. He told reporters he was sorry that his crimes had led to Collier’s death.

“I would definitely like to give my deepest condolences to the Collier family,’’ he told reporters. “I’m very upset the firearm was used to kill him. I had no idea what [Tsarnaev’s] intentions were.’’

“I don’t support it, I don’t support any type of terrorism at all,’’ Silva said. “It’s just sad that a close friend chose to do that with his life.’’

A spokesman for the Collier family said they had no comment on the Silva case.

Tsarnaev, now 22, was sentenced to death in June for his role in the Marathon bombings on April 15, 2013, which killed three people, injured more than 260, and sent the region into a weeklong scare while he evaded capture.

His only known accomplice, his older brother Tamerlan, was killed during the firefight with police in Watertown on April 19, 2013. Authorities said the brothers crept toward Collier’s car in the dark of night on April 18, shot him, and attempted to steal his gun before fleeing the area, after authorities had released photos identifying them as suspects.

The Tsarnaev brothers are the only known masterminds, yet their friends were ensnared in the investigation. Within weeks of the bombings, two of Tsarnaev’s close friends at the University of Massachusetts Dartmouth, where he attended college, were charged with removing evidence from his dormitory room after they identified him in FBI photos, and another one was charged with lying about what he knew about the episode.

Azamat Tazhayakov, who cooperated with authorities and pleaded guilty, was handed a 3½-year sentence in prison, and Dias Kadyrbayev was sentenced to six years.

Another friend, Robel Phillipos, who has known Tsarnaev from their time at Cambridge Rindge & Latin High School, was convicted of lying about seeing the two friends enter the dorm room and was sentenced to three years in prison.

A fourth friend, Khairullozhon Matanov, who was closer to Tamerlan Tsarnaev, was sentenced to 30 months in prison for obstructing the investigation by giving misleading statements to investigators.

Silva’s connection to the Ruger pistol that was used to kill Collier was not clear until about a year after the bombings. The gun was found at the scene of the Watertown shootout, but it had a serial number that had been obliterated.

Several witnesses, authorities said, later told investigators that the pistol had once belonged to Silva, who used it for “protection’’ in his drug-dealing operation. He had been dealing more than $3,000 worth of marijuana a month and had been robbed twice.

Later in 2012, Silva subsequently acknowledged, he lent the gun to Tsarnaev, who said he wanted to rob two drug dealers from Rhode Island. But Tsarnaev never gave the gun back, in spite of Silva’s request that he return the weapon.

Ultimately, Silva was ensnared in a heroin-dealing sting in the summer of 2014 and admitted to once possessing the gun. He testified at Tsarnaev’s trial, and prosecutors said his testimony was instrumental in debunking Tsarnaev’s defense team’s theory that the older brother, Tamerlan, was the mastermind, who acquired the weapon. Silva told jurors that his friend Dzhokhar Tsarnaev was the first to take it.

Silva, who also attended Cambridge Rindge & Latin and was one of Tsarnaev’s closest friends, faced a minimum of five year in prison on the charges of dealing heroin, because of his possession of the weapon. But prosecutors agreed to seek a reduced sentence based on his “substantial assistance’’ in the case.

Wolf told Silva that he recognized the young man within years went from “dealing marijuana to facilitating murder,’’ and the emotional tolls that Tsarnaev’s crimes had on him.

“You will have to live with the memory of your crimes and conduct, I expect forever,’’ the judge said.

Patricia Wen of the Globe staff contributed to this report. Milton J. Valencia can be reached at mvalencia@globe.com. Follow him on Twitter @miltonvalencia.