The for more than half a century, since 1971, unsolved murder of a teacher in Vermont was solved within hours after local authorities linked a discarded cigarette butt to a suspect who became a Buddhist monk after the murder.

The suspect, William DeRoos, died in San Francisco 15 years after he strangled Rita Karan, 24, said Burlington Police Chief Joe Marant.

A combination of genetic genealogy, DNA testing and a recent interview with DeRoos’ ex-wife allowed authorities to identify him as the alleged killer, Marand told reporters.

Karan’s roommates found her dead in their apartment on July 19, 1971. DeRoos lived with his wife two floors above Karan, but never considered a suspect at that time. In an interview with authorities the day after the slaying, DeRoos and his wife, Michelle, gave an alibi, saying they were home the night of the slaying and didn’t hear or see anything, said Jim Tribe, chief of detectives police station.

The case was dropped, but authorities investigating the case in 1971 collected a piece of evidence — a cigarette butt — that became the “key” to solving it, Trimb explained. The object was located, and submitted for DNA processing in 2014. A male DNA profile was found, but no identification was made.

The department partnered with Parabon NanoLabs, a genetic genealogy company that combines historical research with genetic analysis, to search for clues to the killer’s identity.

Keke Moore, the company’s chief genealogist, told reporters Tuesday that, working from a public database, she was able to narrow down the list of possible suspects to DeRouche – within a few hours.

After the discovery, detectives re-interviewed DeRouche’s then-wife, and she revealed to them that on the night of July 19, 1971, she and her husband had argued and he had left. He returned after she had gone to bed and later told her to tell the authorities that he had been home all night.

“I think she lied at the time because she was young, naive, newly married and in love… She didn’t want to get him in trouble for a crime she didn’t believe he committed,” commented a detective who worked on the case.

After the assassination, DeRouche left his wife and moved to Thailand to become a Buddhist monk. By 1974 he was back in the US, living in San Francisco, where another woman he married told authorities she knew him as a guru named “Dutch”.

The woman stated that he could become violent, once he even choked her, while he stabbed a friend in an unprovoked attack.

The woman divorced DeRoos, who died of a drug overdose in a San Francisco hotel in 1989, officials said.

Additional DNA testing found a sample linked to DeRoos’ DNA on a jacket Karan was wearing on the day of the murder.