He was even more confused when an FBI agent asked to swab his cheek for DNA.

"They wouldn't tell me anything," Usry, now 43, tells The Post. "I was like, 'Am I being accused of a crime? Do I need a lawyer?' "

It was only later that Usry, a low-budget filmmaker, learned he was a suspect in the 1996 murder of 18-year-old Angie Dodge. There were a few things that pointed to him: He'd visited Idaho Falls, Idaho, where the victim was killed, during the same time frame of the murder. He had directed a 2010 film called "Murderabilia," which — according to the search warrant — "dealt with some sort of homicide or killings."

But the main reason cops showed up on Usry's doorstep was his DNA. When genetic evidence from the crime scene didn't match anything in the national law-enforcement database, the police ran a familial DNA search on Ancestry.com, the world's largest for-profit genealogy company.

They found a close match between semen found at the murder scene and the DNA of Usry's dad, who had donated his saliva to Ancestry as part of a genealogy project with his church. When the elder Usry was deemed too old to be a suspect, it led detectives to his son. Although the younger Usry had never used Ancestry or any other genealogy service, his dad's DNA was enough for a judge to issue a warrant.

Even after Usry's DNA was tested and his name cleared — which took weeks — he didn't rest easy.

"I know that my personal genetic information is still in the FBI criminal database," he said. "They're not going to just throw it away because of their mistake. But what are they doing with it?"

Use of direct-to-consumer DNA tests have exploded over the past decade, with an estimated 100 million people worldwide sharing their genetic information with companies like AncestryDNA, 23andMe, and FamilyTreeDNA. All that DNA has been a boon to law-enforcement agencies, who've discovered that "investigative genetic genealogy" is far more effective if searches aren't restricted to DNA left at a crime scene.