A 62-year-old man whom authorities in December identified as the killer of a radio intern 40 years ago in Highlands Ranch pleaded guilty to first-degree murder Friday in Douglas County Court.

James Curtis Clanton was arrested in December in connection with the 1980 stabbing of 21-year-old Helene Pruszynski — a case that stayed cold for decades before DNA on a secretly snagged beer mug led to a breakthrough last year. The case is the latest success in the growing field of using genetic tools to solve decades-old crimes.

Clanton is set to be sentenced April 10.

Detectives also used DNA from semen collected at the scene, connecting Clanton to the crime using online genealogy databases and help from private companies.

Pruszynski, a college senior from Massachusetts interning at KHOW radio in Denver, had only been in the state a few weeks when she was killed.

She didn’t come home Jan. 16, 1980. Her body was discovered dumped in a field the next day, with nine stab wounds in her back and evidence that she had been raped repeatedly.

The case flummoxed investigators for decades until the DNA breakthrough. Around Thanksgiving last year, detectives followed Clanton to a Florida bar, secretly snatching his empty beer mug to collect his DNA.

The DNA matched a sample taken from the crime scene.

“Because of the unrelenting and outstanding efforts of the Douglas County Sheriff’s Office and United Data Connect, the resolution of a horrible sexual assault and murder in a desolate part of our county four decades ago ended within 15 minutes inside a courtroom this morning,” George Brauchler, the 18th Judicial District District Attorney, said in a news release.