Old-fashioned detective work would have solved a murder that led to the arrest of a serial killer in Philadelphia, but technology left the perpetrator without a defense, police said.

"We would have been able to solve the case without technology," said Homicide Unit Det. Charles Boyle. "It was a tip that did it, but the technology made it a slam-dunk."

Boyle, who investigated the shooting of 48-year-old Patricia McDermott in May 2005, said a tipster identified Juan Covington as a suspect, but video collected from more than 50 private and public cameras brought the tipster forward and strengthened the case. It was not the first time authorities have used technology in a criminal investigation, but it was the first time detectives in Philadelphia used so many surveillance systems to solve a crime, Boyle said.

After a driver found McDermott's dead body, police spent several weeks collecting and reviewing video from businesses, apartment buildings, parking lots and public property to trace the victim's steps. Images from cameras installed around a post office showed the killer following McDermott off a bus during her morning commute, shooting her in the back of the head and running away.

Using the killer's direction of travel as a guide, police went to neighboring properties in search of more videotape. Several -- some using infrared technology and others with poor quality footage -- showed the perpetrator escaping before dawn.

Since none of the images provided a clear picture of the perpetrator's face, police released the images to the public, hoping for a tip. One of hundreds of tipsters told police the image resembled Covington, a regular on the bus McDermott had boarded.

Police soon discovered that Covington worked at the same hospital where McDermott worked as an X-ray technician. That day, investigators found surveillance video of Covington showing up at his workplace -- in the same clothing -- soon after the murder.

"It's a shame that it's after the fact, but it's a pretty powerful tool," Boyle said.

Covington told police he believed McDermott was deliberating exposing him to radiation on the job. The 44-year-old admitted to shooting four other people, two of whom survived. He pleaded guilty in 2005. Now, he is serving a life sentence in prison.

Boyle said police probably could have caught Covington through interviews or other evidence.

"People convict themselves in the end," he said. "They tell one story, and things don't match up."

Without the video, Covington may have had a better defense, Boyle said. So, clues from technology -- including cell phone records, transit card bar codes and credit card purchases -- often help.

Without the video from the crime scene and escape route, police investigating the murder in Philadelphia could have requested information on every transit card swiped on the bus after McDermott had boarded. They could have traced that information back to individuals through credit card purchases. They also could have traced bar codes from swiped transit cards to the times and locations of transactions conducted by passengers on the bus.

"Then we would pray that they've got a video," Boyle said.