Police detectives said they were only humoring a man who flagged down their unmarked car and offered them an unsolicited DNA sample.
Then they got a surprise from the crime lab.
The sample matched an unrelated rape case from 2000, investigators said, leading to charges against the St. Louis man.
Christopher Swift, 30, is charged with rape, sodomy, kidnapping, second-degree robbery and felonious restraint.
He is accused of an attack on a woman, then 42, who told police she fell asleep in her car and woke up to find a man raping her. Until Swift's DNA results came in, police had no suspects.
"This was certainly an unusual series of events that led to a suspect in an old case," said Capt. James Gieseke, commander of the Crimes Against Persons Division. "We were looking for a suspect in another series of crimes and ended up identifying a suspect in an unrelated crime."
The unexpected break came in June, as sex crimes detectives Rick Noble and Mark Chambers were searching for suspects in a series of home invasions. As they met young men who fit the suspect's description, they would ask for a swab of his DNA to compare against material found at the crime scenes.
The two saw Swift leaning into a car and talking to a woman inside, police said, but they moved on because he did not match the description.
Shortly after that, they spotted him running after their car, angrily demanding to know why they had been staring at him. Swift had been drinking, police said.
The officers identified themselves and told Swift what they were doing, police said, and he calmed down. They sent him on his way, but he insisted they take a DNA swab.
The hit from the crime lab came in mid-July.
Swift told The St. Louis Post-Dispatch this week that the police asked for the swab but agreed that he willingly provided it.
"I had had a couple of beers. I told them, 'Go ahead and take it,'" he said in an interview at the City Justice Center. "I don't have anything to hide.'"
Swift, who police said has no record of felony arrests, said he is innocent of the rape.
"This is crazy. I don't have the slightest idea how they got that match," he said. "I can't explain it. Some mistake must have been made."
Not long after their encounter with Swift, police made an arrest in the series of five home invasions.
Keith A. Nabors, 21, was arrested June 7 after an informant turned him in.
Nabors, who police said has confessed to the crimes, is charged with rape, attempted rape, sodomy and numerous burglary- and stealing-related charges.