Surveillance footage taken of 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 footage captured by a security camera 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 4th Street and Girard.

A man can be seen biking in the opposite direction, passing O'Donnell, then doubling back. But police don't know whether the man is connected to the Wednesday killing, and investigators are hoping they can enhance the quality of the footage.

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

Investigators also have recovered DNA from the scene of O'Donnell's murder, and are analyzing it in hopes that it might lead to her killer. Clark would not elaborate on 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 was sexually assaulted, and died of strangulation. She was also beaten on her head and body.

Investigators now believe O'Donnell was killed sometime between 2 and 3:45 a.m. on Wednesday morning, 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 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 naked body shortly before 10 a.m. Wednesday, partially placed beneath some 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 O'Donnell's killer knew her somehow. And though investigators do not believe O'Donnell was followed home from the bars she visited Tuesday night, Clark said it was possible her attacker had seen her around the neighborhood before.

People are calling the police with tips, Clark said.

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

O'Donnell's killing has jolted residents of the increasingly upscale Northern Liberties neighborhood, which has 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 help walk them home safely.

Meanwhile, 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 profits from door sales to her family. Tommy Updegrove, owner of PYT, where she 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.