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Police can use a person’s abandoned sweat and saliva to create a DNA profile based on nothing but a tip from genealogical analysts, a judge in Alexandria Circuit Court ruled Thursday — the first case of its kind in Virginia state courts.

Jesse Bjerke was first identified as a likely suspect in the 2016 rape of an Alexandria lifeguard when researchers ran DNA from the crime scene through public genealogical databases. After building a family tree, they narrowed their search to Bjerke; police followed him and lifted his DNA off straws he left at an Old Town restaurant.

Judge Lisa Kemler rejected a defense argument that the search was unconstitutional without a warrant and that the genealogical report wouldn’t be enough for police to get one.

“The items that contained Mr. Bjerke’s DNA were abandoned property,” Kemler said, and the police use of that DNA “did not reveal private, personal information other than his identity as a suspect in this case.”

She agreed with Assistant Commonwealth’s Attorney Jessica Smith that DNA as used by police is akin to a fingerprint — an invisible marker of identity. Defense attorneys had countered that DNA is more akin to a cellphone, filled with sensitive personal information that in most cases cannot be accessed by police without a warrant.

Even if a higher court disagrees, Kemler ruled that the police acted in good faith and thus the evidence can be used against Bjerke.

Defense attorney Chris Leibig urged the court to look not just at what happened in this case but at what could happen in the future.

DNA shouldn’t be considered abandoned, he contended, because it is impossible to avoid leaving it everywhere, as Moses Schanfield of George Washington University’s Department of Forensic Science testified.

“Everything we touch, we leave a DNA trail,” he said.

“DNA is a treasure trove of information just sitting there and there’s nothing preventing them from doing anything with it,” Leibig argued. “There needs to be a rule” that isn’t “pure trust in the government.”

The judge replied that based on how DNA is currently used “we’re not there yet,” although she acknowledged that the law in this area is “in flux.”

Genealogical research has become an increasingly common tool for solving cold cases, and there is no legislation guiding how law enforcement can use genetic data.

Virginia’s state forensic lab looks at parts of DNA that can currently be used only to illuminate a person’s identity and family relationships. Parabon NanoLabs, a Reston-based company that conducted the genealogical research, looks at far more. But Smith argued that Bjerke had no claim over the anonymous crime-scene DNA sent to Parabon for analysis. Police, she said, would have no reason to have his DNA analyzed further by the company once he was identified.

Detectives on the case first employed Parabon in 2016, when the company created a composite image of the suspect based on his DNA. That image did not prove useful, but three years later the company reached out to police again to say they had a new and more powerful tool — genetic genealogy.

Smith noted that police had similarly acted on a tip from the community to surreptitiously test DNA from an abandoned cigarette smoked by a man whose mother suspected him of committing a similar rape in Fairfax County. The mother told a friend who went to police. It was not a match.

Kemler asked prosecutors whether authorities could legally collect abandoned DNA from everyone in an apartment building where a crime had occurred. Smith said they could but that it was unlikely they would.

“Police are never going to insufficiently use their time,” she said.” They’re not going out and collecting random DNA.”