This photo from the time of his arrest in January 2014, shows David DiPaolo of Bristow. He was charged with voluntary manslaughter in the death of Geoffrey Farrar. (Warren County Sheriff’s Office)

A Virginia rock climber pleaded guilty on Monday to killing the climber who was his mentor for 20 years, by hitting the man on the head with a claw hammer.

David DiPaolo, 33, will likely be sentenced to 10 to 15 years in prison, in accordance with the plea agreement, the U.S. attorney’s office in Maryland said in a statement on Monday.

He was charged with voluntary manslaughter for killing Geoffrey Farrar, 69, at the Carderock Recreation Area in Bethesda in December 2013.

Farrar was such a fixture at the climbing area that he was known as “Carderock Geoff.” He tried out risky climbs and taught younger climbers, including DiPaolo, who became his sidekick “Carderock Dave.”

When fellow climbers found Farrar bleeding from his head at the base of a cliff near the popular Billy Goat Trail, they thought the avid rock climber had fallen.

But authorities said he was beaten to death by DiPaolo.

DiPaolo, who lives in Bristow, initially said he acted in self-defense.

Authorities agree with DiPaolo on how the encounter started: The two men, who first met when DiPaolo became interested in climbing at age 11, argued about something in the parking lot on Dec. 28, 2013.

Farrar went to the base of the climbing area, and DiPaolo went to his car. Then DiPaolo came back and found Farrar. DiPaolo was carrying his claw hammer, a climbing implement that soon became a deadly weapon.

DiPaolo fled to New York state after the killing. When he was arrested on Jan. 8, 2014, he told police that Farrar had been choking him and he had struck the older man in self-defense.

According to court documents, DiPaolo told police, “I’m sorry this happened. I didn’t want it to happen. I didn’t know it was going to happen.”