Surveillance cameras in Northern Liberties and Fishtown last week captured images of a man who biked in aimless circles for almost an hour before he crossed paths with Sabina Rose O'Donnell then appeared to follow her home in the moments before she was killed.

"We think he was on the prowl, looking for a victim," Capt. James Clark of the Philadelphia Homicide Unit said Thursday. "We believe that someone knows who he is. We're hoping someone will call us with that information."

Clark described the man Thursday as a person of interest and said he believed the man lives in O'Donnell's neighborhood.

Police released camera footage taken shortly before 3 a.m. on June 2 that caught O'Donnell, 20, pedaling north on Fourth Street and crossing Girard Avenue on the way to the apartment she shared with her stepfather. She passed the man as he biked in the opposite direction, and almost immediately the man made a U-turn and doubled back after her.

Police have not released a detailed description of the man, but they said those who know him might recognize him from the footage.

The man on the tape is wearing jeans and a white tank-top-style undershirt, biking up and down mostly empty streets. Shortly after 2 a.m., he passes a store at the corner of Front Street and Girard, and later pedals west on Girard at Fifth Street.

Authorities said O'Donnell was out with friends in Northern Liberties from 7:30 to 10:30 p.m. June 1, then went to a friend's apartment at Front Street and Girard. She left between 2 and 2:30 a.m., Clark said.

According to the footage, O'Donnell crossed paths with the man on the bike around 2:48 a.m. Police believe O'Donnell reached the front of her apartment building but was snatched off her bike and dragged around the building to a vacant lot in back.

A passerby found O'Donnell's nude body shortly before 10 a.m. the next day. She had been strangled, beaten, and sexually assaulted.

Investigators are analyzing evidence collected from the scene, including DNA material and a man's white shirt. Detectives also have pored through hours of surveillance footage from various sources.

A $25,000 reward has been posted for information leading to the arrest and conviction of the man responsible for the death of O'Donnell, a well-known member of the Northern Liberties community who worked as a waitress in a restaurant at the Piazza at Schmidts. Friends and local businesses have raised money for her family and are talking about building a permanent memorial to her.

Police emphasized that the reward money could be collected anonymously.

"Once we have the public's help, the physical evidence will convict this person," Deputy Police Commissioner William Blackburn said. "We just need to be steered in the right direction."

Less than an hour after police released the video, more than 200 people gathered for a memorial service at Liberty Lands, where O'Donnell used to hang out with friends, walk dogs, and play with ladybugs.

"People keep saying, 'Sabina's in a better place.' But I know any place that Sabina's in is a good place," said Rachel O'Donnell, her mother.

Talea Bella, 28, of Old City, was the first of many performers who took the stage in the park to honor O'Donnell, an aspiring dancer.

Bella said she became choked up as she sang an a cappella rendition of "Somewhere Out There," a ballad from the 1986 animated movie An American Tail.

"Nobody could have thought something like this could have happened to such an innocent, sweet, and uncorrupted girl," Bella said. "Everybody wanted to be friends with her."

Anyone with information about O'Donnell's killing can call police at 215-686-3334 or leave an anonymous tip at 215-546-8477.

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