DENVER (Reuters) – A man who prosecutors say planted a bomb outside a small-town Colorado police station to avenge a friend’s murder nearly a half-century ago pleaded guilty on Tuesday to federal charges that could send him to prison for the rest of his life.

David Michael Ansberry, 65, pleaded guilty in U.S District Court in Denver to attempted use of a weapon of mass destruction in connection with the botched bombing of the Nederland, Colorado, police station, a spokesman for acting U.S. Attorney Robert Troyer said.

The improvised explosive device failed to go off despite multiple attempts by Ansberry to detonate it with a cellphone, according to an FBI arrest warrant affidavit.

According to a memorandum prosecutors filed before the hearing, Ansberry belonged to a nomadic group of hippies called “Serenity, Tranquility and Peace,” or STP.

The itinerant group had a presence in Nederland, a mountain community about 15 miles west of Boulder, in the late 1960s and early 1970s. One of STP’s members, Guy Goughnor, was slain there in the summer of 1971.

“The Nederland town marshal, Renner Forbes, eventually confessed to killing Goughnor and was convicted for that crime in 1998,” federal prosecutors wrote in court documents. Forbes has since died.

In October 2016, a Nederland police officer discovered the bomb in a backpack, prompting the evacuation of the police station and nearby buildings before explosives experts disarmed the device.

Goughnor went by the nickname “Deputy Dawg” and a message was scrawled at the attempted bombing scene which read, “RIP Deputy Dawg and the date 7-17-71,” prosecutors said.

Authorities said Ansberry had harbored a grudge for more than four decades and traveled from California to Idaho, where he purchased bomb making components before arriving in Colorado.

Detectives were led to Ansberry after tracing the purchase of cellphones to a Colorado grocery store where security cameras captured images of Ansberry. He was easily identified by his diminutive 3-foot, 6-inch stature.

Federal agents arrested Ansberry at Midway Airport in Chicago as he was preparing to catch a flight to Baltimore, and returned him to Colorado.

Ansberry’s public defenders did not immediately respond to a message seeking comment, but the Denver Post reported that during Tuesday’s hearing, Ansberry said he pleaded guilty to avoid additional charges.

He faces up to life in prison and a maximum $250,000 fine when he is sentenced in November.