By Maria Aguilar Prieto, CNN
(CNN) — More than four decades after a teenager was murdered in California, DNA found on a discarded cigarette has helped authorities catch her killer.
Sarah Geer, 13, was last seen leaving her friend’s house in Cloverdale, California, on the evening of May 23, 1982.
The next morning, a firefighter walking home from work found her body, the Sonoma County District Attorney’s Office said in a news release. She had been dragged down an alley to a secluded area near an apartment building and behind a fence, where she was raped and strangled, according to authorities.
Her death was ruled a homicide, but due to the “limited forensic science of the day,” no suspect was identified and the case went cold for decades, prosecutors said.
Nearly 44 years after Sarah’s murder, a jury found James Unick, 64, guilty of killing her on February 13. It would have been the victim’s 57th birthday, the Sonoma County District Attorney’s Office told CNN.
Genetic genealogy, which combines DNA evidence and traditional genealogy, helped match Unick’s DNA from a cigarette butt to DNA found on Sarah’s clothing, according to prosecutors.
“This guilty verdict is a testament to everyone who never gave up searching for Sarah’s killer,” District Attorney Carla Rodriguez said in the release. “This is the coldest case ever presented to a Sonoma County jury. While 44 years is too long to wait, justice has finally been served, both to Sarah’s loved ones as well as her community.”
A DNA profile built
A break in the case first came in 2003, when investigators developed a DNA profile based on sperm collected from Sarah’s underwear, prosecutors said.
However, the profile did not match anyone whose DNA was available for comparison in law enforcement databases at the time, according to the release, and the investigation came to a halt again. Those databases include information of known criminal offenders.
In 2021, the Cloverdale Police Department reopened the investigation into Sarah’s death. The department said it had been in communication with a private investigation firm in late 2019 and had partnered with them in hopes the firm could revisit the case’s evidence “with the latest technological advancements in cold case work.”
The investigation also enlisted the FBI to help identify a potential match to the 2003 DNA profile.
“The FBI, with its access to familial genealogical databases, concluded that the source of the DNA evidence collected from Sarah belonged to one of four brothers, including James Unick,” prosecutors said.
Genetic genealogy cracks the case
Once investigators narrowed down the list of suspects to the four Unick brothers, the FBI “conducted surveillance of the defendant and collected a discarded cigarette that he had been smoking,” prosecutors said.
A DNA analysis of the cigarette confirmed James Unick’s DNA matched the 2003 profile, along with other DNA samples collected from Sarah’s clothing the day she was killed.
Investigators were able to crack the case thanks to the emerging field of genetic genealogy, which combines DNA analysis and family tree research.
Essentially, a DNA sample is compared to publicly accessible databases of millions of people who have contributed their genetic profile, and investigators can then piece together a family tree that leads to a suspect.