PHILADELPHIA—Surveillance video capturing the final minutes of a police officer's life shows him repeatedly looking over his shoulder toward two men following him before he was shot and killed.

Philadelphia police posted the video online Monday with the hope that someone might recognize the men in the blurry images and contact authorities.

Officer Moses Walker Jr., 40, was slain during a stickup by two men as he walked from his north Philadelphia police precinct to a bus stop at around 6 a.m. Saturday. He had just finished his shift and wasn't in uniform, clad instead in shorts and a baseball cap and a large backpack on his shoulders.

"He looked like a young civilian walking down the street, and we think that's why he was targeted," said Capt. James Clark, head of the homicide unit.

The four-minute video shows Walker looking over his shoulder several times as he walks along a streetlight-illuminated sidewalk, followed by grainy footage of two men walking behind him, first on the opposite side of the street and then crossing over. Out of the camera's view further down the block, the 19-year police department veteran was shot.

"I realize we don't have a real clear view of the suspects, but someone may know who they are just by clothing, the way they walk, the fact that they may hang around at that time of morning in that area," police Commissioner Charles Ramsey said. "Let us decide if any information you have is relevant. Do not decide that for yourself."

A third man in the video, who police believe had no involvement in the crime, was a witness to the shooting and gave police some details about what happened.

"Two males approached (Walker), one of which had a gun, a robbery was announced and he demanded money," Clark said. "During this time, the officer attempted to pull out his off-duty weapon ... one of the males fired, striking him three times: once in the chest, once in the stomach and once in the hand."

Clark said three or four armed holdups fitting a similar pattern have occurred in the area over the last couple of months and added that the killers probably live near the scene of the crime.

"We know people out there know who they are," he said.

A reward for information leading to the arrest of Walker's killers continued to rise as public and private donations poured in, reaching $88,000 by early Monday evening.

Authorities brought in one man for questioning Monday after stopping his car and arresting him on an outstanding warrant for a recent armed robbery in the neighborhood. They were also trying to track down several other men for questioning who are known to have robbed people in the area, Clark said.

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Online:

Surveillance video: http://www.phillypolice.com/news