Rita Curran, a 24-year-old Irish American woman from Vermont, was found murdered in her apartment in July 1971.

Now more than fifty years after her death, a discarded cigarette butt at the crime scene has helped to solve the decades-long cold case.

DNA evidence lead investigators to Curran's upstairs neighbor, who police say strangled the young elementary school teacher after he had a fight with his wife.

Burlington Police Department (BPD) announced the identity of Curran's suspected killer as William DeRoos, who was 31 at the time of the murder, reports Fox News.

During a press conference last week, BPD Chief Jon Murad said DeRoos moved to Thailand and became a Buddhist monk shortly after the murder.

According to the police chief, DeRoos died of morphine poisoning in San Francisco in 1986, when he was 46, about 15 years after murdering Curran.

DeRoos lived two floors above Curran with his wife, Michelle. The couple had only been married two weeks when they got into an argument and DeRoos left for "a cool down walk," during which time investigators believe he killed Curran.

When he returned home, he told his wife not to tell anyone that he had been out, reports CBS News.

Kindergarten teacher Rita Curran was raped and murdered in 1971. She was 24. When I left Burlington, finding her killer was unfinished business. I'm so proud of the @OneNorthAvenue detectives who closed her case today. Thank you, all. The police never forgot about you, Rita. pic.twitter.com/ahhcTSGO3G

— Brandon del Pozo, PhD, MPA, MA (@BrandondelPozo) February 21, 2023

According to Murad, the initial incident report stated that Curran "put up a vicious struggle" with the suspect and "fought for her life." 

Curran's body was discovered by her roommates in the early morning hours. When police questioned the neighbors, both DeRoos and his wife denied witnessing anything suspicious the night before.

The investigation was renewed in 2019 and when the police interviewed DeRoos's former wife again, she told them he had convinced her to lie at the time and had in fact left the apartment that night.

BPD Lt. Det. James Trieb said that investigators had discovered an un-crushed cigarette butt next to Curran's body at the crime scene.

"That cigarette butt sat in evidence for over 40 years" until 2014, when it was sent to the New York City Office of the Chief Medical Examiner for processing.

Researchers discovered a single, male DNA strand on the cigarette and compiled a DNA profile of the smoker. However, the profile did not match and samples in the national criminal DNA database.

When the case was reopened in 2019, investigators contacted Parabon Nanolabs Chief Genetic Genealogist CeCe Moore for additional assistance identifying the DNA strand, and it finally resulted in a match this year.

According to CNN, the DNA was analyzed using "genetic genealogy," which looks at DNA databases used for genealogy research by the public to compare genetic material of the suspect with possible family members.

The sample, which had been traced through relatives on both sides of DeRoos's family, pointed at DeRoos, even though he had no DNA profile on record.

Investigators then found a living half-brother of DeRoos who was willing to provide a DNA sample to conclude that the cigarette belonged to DeRoos.

Traces of DeRoos's DNA was also discovered on Curran's housecoat.

"This case is over 50 years old, and it only took a few hours to narrow it down to William DeRoos," said Moore during the press conference. 

BPD Lt. James Trieb said: "We're all confident that William DeRoos is responsible for the aggravated murder of Rita Curran, but because he died in a hotel room of a drug overdose he will not be held accountable for his actions, but this case will be closed."

Curran's parents died without knowing who killed their daughter, but her brother and sister were at the press conference to thank BPD.

"My mother came here from Ireland and my father from Newfoundland. We were an old-fashioned, strong, Catholic family. I don't think so much about the guy who did this as I do about Rita and my parents and what they went through. I pray to my parents, and I pray to Rita," said Curran's brother, Tom.

He said his sister hadn't lived on her own for more than two weeks before she was murdered.