Surveillance-camera footage that captured 20-year-old Sabina Rose O'Donnell shortly before her death shows a man apparently following her on the street, police said Friday, but it's unclear whether he can be linked to her killing.

The images from a security camera taken on the night O'Donnell died shows her biking along Girard Avenue, apparently on her way home to the apartment she shared with her stepfather at Fourth Street and Girard Avenue.

A man on a bike passes her in the opposite direction, then doubles back. But police don't know whether he is connected to the Wednesday killing, and investigators hope to enhance the quality of the image.

"It's nothing definitive right now," Philadelphia Police Capt. James Clark said.

Investigators also have recovered DNA from the crime scene and are analyzing it in hopes that it could lead to her killer. Clark would not say what type of DNA material was found, and the analysis will not be completed for several weeks.

The autopsy on O'Donnell, a popular waitress at a Northern Liberties restaurant, confirmed that she had been sexually assaulted and died of strangulation. She also had been beaten on her head and body.

Investigators now believe O'Donnell was killed between 2 and 3:45 a.m. Wednesday, Clark said.

O'Donnell spent the evening in two Northern Liberties bars, then went to a friend's apartment at Front Street and Girard. At some point after midnight, she left the apartment, then biked a few blocks toward home. Police believe a stranger accosted her in front of her home, then dragged her to a vacant, grassy lot behind the building.

A passerby found her nude body shortly before 10 a.m. Wednesday, partially placed beneath bushes with a piece of her clothing wrapped around her neck.

Police have said O'Donnell was killed by someone she did not know, but they have not discounted the possibility that the killer may have known her somehow. And though investigators do not believe O'Donnell was followed from the bars she visited Tuesday night, Clark said it was possible her attacker had seen her around the neighborhood before.

People are calling with tips, Clark said.

"The community has been very helpful," he said. "We are tracking down every lead they give us."

Police on Friday night checked for a suspect at two sites, one in North Philadelphia and one in Francisville, just north of Spring Garden. An officer at the Francisville site confirmed they were searching for a suspect in the O'Donnell killing, but no arrests were made.

O'Donnell's death jolted residents of the increasingly upscale Northern Liberties neighborhood, which has been gentrified so dramatically in recent years that many have come to think of it as safe. In addition to online tributes to O'Donnell, neighbors have begun organizing "buddy-system" programs aimed at helping people find others who can walk them home safely.

Support for O'Donnell's family has continued pouring in from the Northern Liberties community where O'Donnell spent much of her time. A $25,000 reward has been posted for information leading to the arrest and conviction of her killer. Local restaurants and nightclubs have offered door proceeds to her family. Tommy Updegrove, owner of PYT, where O'Donnell worked, started the Sabina Rose O'Donnell Fund with the goal of defraying burial costs and adding to the reward.

One friend has also proposed turning the two lots where O'Donnell was found into the Sabina Rose Gardens, an idea that has drawn more than 1,000 supporters on Facebook.

Anyone with information about the case can call 215-546-8477 and leave an anonymous tip. To donate to the Sabina Rose O'Donnell Fund, visit http://pytphilly.com/sabina/.

Contact staff writer Allison Steele at 215-854-2641 or asteele@phillynews.com.