The investigation into a series of rapes in Pennypack Park was years old, and still open, when a man named Robert Palen was arrested in Wisconsin late last year.
The 38-year-old, born and raised in Philadelphia, had lived near the park, where three women said they had been abducted, beaten, and raped by a man driving a white work van, in 2010 and 2011.
But DNA evidence - found in just one of the cases - never turned up a match, and the trail had grown cold by December 2013, when Palen was arrested and accused of beating, strangling, and raping a Madison, Wis., woman in her home.
Palen pleaded guilty. His DNA was entered into a national database.
In March, a Philadelphia detective called the Madison Police Department. Philadelphia police had found a match.
This week, Palen was extradited to Philadelphia and charged with rape, attempted murder, aggravated assault, and simple assault in two of the Pennypack Park cases.
Authorities were unable to continue the investigations into the case of the remaining victim because she had died, Lt. Anthony McFadden said.
The arrest concluded what had been a citywide investigation launched in 2011, when a 24-year-old woman told Philadelphia police she had been beaten and raped in Pennypack Park. Investigators started looking for other crimes that might fit the same pattern - standard practice for investigations in the Special Victims Unit, McFadden said.
In this case, what detectives found - a nearly identical case from more than a year before - would set off a hunt for a suspected serial rapist.
Detectives patrolled the area, searching for anyone who matched the alleged attacker's description. Vice squads staged prostitution stings. Police officers across Northeast Philadelphia were told to keep an eye out.
A composite sketch was released, and a third victim came forward after recognizing the man depicted in it. But leads were few and far between.
"We did all we could do," McFadden said, before the case "went completely cold. And we didn't have anything."
In March, when Palen's DNA was found to match evidence from the Philadelphia case, the dormant investigation into the park rapes was relaunched. Detectives were able to find and interview the two surviving victims, as well as Palen's employers and family, and trace the work truck police say he used to abduct his victims.
"DNA just helps us," McFadden said.
Palen, who had been jailed in Wisconsin since his conviction in the sexual-assault case there, is awaiting a preliminary hearing in Philadelphia.
"He is a brutal individual," McFadden said. If not for his arrest in Wisconsin, he added, "this would still be an unknown offender."