Philip Duran has pleaded guilty to providing a handgun to a juvenile and illegally possessing a sawed-off shotgun. Prosecutors dismissed a charge of contributing to the delinquency of a minor.
Duran could get up to nine years in prison when he's sentenced on June 23.
The TEC-DC9 assault handgun Duran provided was one of four weapons teenagers Eric Harris, 18, and Dylan Klebold, 17, used April 20, 1999, to kill 13 people at their Littleton, Colo., school before taking their own lives.
Authorities said Klebold fired off 55 rounds from the TEC-DC9 during the shooting rampage at the school, killing four people and wounding two others.
Jefferson County prosecutor Steve Jensen said Duran knew Harris and Klebold when they worked together at a pizza parlor, and he introduced them to a gun seller at a gun show three months before the attack.
The gun dealer, Mark Manes, pleaded guilty and was sentenced to six years in prison in November for providing a handgun to the minors.
The prosecutor said he would not make any recommendations on what Duran's sentence should be, other than to urge the judge to impose "an appropriate sentence."
He would not comment on whether he consulted with shooting victims' families about the plea agreement. But he said he is pleased to close another chapter related to Columbine.
"I'm glad he pleaded guilty. I'm glad he had the integrity to do it," Jensen said after the hearing.
Neither Duran, who remains free on bail, his attorney nor his parents spoke with reporters as they left the courtroom.
Harris and Klebold obtained the other weapons they used in the rampage by talking a friend, Robyn Anderson, into getting three rifles for them at a local gun show while they were still minors. But in Colorado, where ranching and hunting are part of a traditional lifestyle, it is not against the law to give a minor a rifle.
The Colorado state legislature passed a bill in the session that just ended that makes it illegal to furnish a handgun to minors without the consent of their parents. The governor is expected to sign the bill.