In the old days, detectives at a crime scene would scour the area for witnesses and physical evidence.

Now they also look for something else, and usually they find it: security cameras.

The growing number of cameras installed to enhance the safety of public buildings and private homes is also providing police with invaluable evidence.

"The technology has definitely evolved," said Lt. Frank Vanore, a Philadelphia Police Department spokesman. "It's becoming more important, and we're getting better at working with it."

Modern homicide detectives don't just look down, searching the street for shell casings and blood spatters - they look up, for cameras that might have captured the crime.

Camera footage helped lead police to 18-year-old Donte Johnson, charged this week with killing Sabina Rose O'Donnell, 20, a popular Northern Liberties waitress. She was bicycling home from a friend's house but never made it, her body found June 2 in an overgrown lot behind her apartment at Fourth Street and Girard Avenue.

In the days afterward, Northern Liberties residents provided police with a list of cameras at local businesses and apartment buildings. Soon, detectives were working around the clock, viewing hundreds of hours of footage taken by dozens of cameras.

Cameras caught Johnson as he spent at least an hour biking around Northern Liberties and Fishtown. Police eventually focused on footage from three. One showed O'Donnell biking past Johnson on Fourth Street, and him making a U-turn to follow her.

One recording provided a clear view of Johnson's face. The day after it was released to the public, police got a tip that led to him.

Experts who study the use of security cameras in American society say that at this point, criminals ought to know better. Cameras are everywhere.

People are routinely recorded when they extract money from an ATM, drive through a toll booth, stroll past a business, or even take a young lady into a hotel room - as discovered by Joran van der Sloot, charged with murdering 21-year-old Stephany Flores in Peru and long suspected in the disappearance of American student Natalee Holloway in Aruba.

Across the United States, colleges and transportation agencies - in Philadelphia, including SEPTA and the University of Pennsylvania - typically allow police to tap into their network of cameras.

"People forget that we're always under surveillance, except maybe in our home," said Tod Burke, a criminal-justice professor at Radford University and a former Maryland police officer. "Once we walk out those doors, everyone can now see what we're doing and record what we're doing."

Society has drawn the line at putting cameras in store dressing rooms and public bathrooms, he said, but most other areas are open to video recording by police agencies, businesses, or citizens.

"Most of the time, we no longer think about it," he said. "We've become so accustomed to being a video society."

Surveillance footage from inside the posh Piazza at Schmidts in Northern Liberties captured what police called the "organized, calculated" shootings of Rian Thal, 34, and Timothy Gilmore, 40, in 2009. Three gunmen hid in the stairwells near Thal's apartment, ambushed the pair, and fled.

Investigators believe Thal and Gilmore were targeted because of their involvement with drugs. The trial of eight people arrested in connection with the slayings has been set for November 2011.

Camera footage also helped lead to the apprehension of Craig Arno and Jessica Kisby, charged in the May 21 carjacking and killing of casino patron Martin Caballero.

Caballero, a grocery store manager from Hudson County, N.J., had gone to the Taj Mahal Casino Resort with his wife to celebrate their daughter's 22d birthday. After dropping his wife at the entrance, Caballero, 47, drove to the Taj garage.

A camera captured the entrance of his Lincoln MKS, followed by a Toyota. Minutes later, surveillance video showed a man and woman approaching Caballero, who had parked. A few minutes after that, a third camera recorded the Lincoln pulling out, followed by the Toyota.

Those shots were the first of nearly a dozen taken over four hours that helped authorities track the suspects' movements. One camera showed Arno withdrawing $300 with Caballero's ATM card.

In Camden, surveillance cameras helped authorities make arrests in two recent homicides.

In 2008, Daniel Benvenutti, 28, of Camden, was shot to death on Federal Street in front of a travel agency after a fight with another man. The agency's camera captured the shooting. In 2009, authorities released video from a robbery at Alex's Bakery in Woodlynne that led to the death of the owner. Three suspects were arrested.

"When video is available, it can be enormously helpful," said Jason Laughlin, a spokesman for the Camden County Prosecutor's Office.

Civil rights advocates have long worried that the widespread installation of cameras impinges on Americans' freedom, their use more appropriate for a police state than a bastion of liberty. Two years ago, for instance, students in Ann Arbor, Mich., organized to oppose the installation of security cameras at Pioneer High School.

For police, the issue is not just legal but technical. Philadelphia investigators often find themselves trying to wrest information from unfamiliar software and a variety of recording devices. Sometimes, police end up taking the entire system to headquarters to ensure nothing is lost.

Currently, the Police Department is able to view footage from more than 600 cameras throughout the city, including lenses around City Hall.

"It's not like years ago when we would just bring a VHS tape back to headquarters," Vanore said. "You have to look at these videos for hours, just to find that one second where someone's going by."