The FBI is assisting with the murder investigation through a joint task force on violent crime that the agency maintains with Philadelphia police, an FBI spokesman said.
Capt. James Clark, commander of the Homicide Unit, said investigators believed the bandits lived in the neighborhood.
Investigators were out in force looking for residents with robbery records to see what, if anything, they might know about the deadly stickup.
Among those homicide detectives planned to question was a fugitive who was arrested on a robbery warrant after police stopped his car at 24th and Edgley Streets, several blocks from where Walker, 40, was slain.
Police Commissioner Charles H. Ramsey urged residents to call if they had the slightest suspicion about the identities of the two males shown in the video.
"Let us decide if any information you have is relevant," Ramsey said at the news conference.
The video, one of several examined by investigators, shows Walker strolling west on Cecil B. Moore Avenue at North 18th Street about 5:55 a.m. to catch a bus home after completing his shift at the nearby 22d District.
The 19-year veteran of the force, who was dressed in shorts, wearing a baseball cap, and carrying a backpack, can be seen glimpsing over his shoulder across the street.
A second camera view shows him walking into the street, around a car, and back to the sidewalk, apparently wary of the two males trailing him.
The same camera view then catches the two males coming up on the other side of the street. They crossed the street and followed Walker.
A third man, whom police have interviewed, also crossed the street and followed the pair.
Clark said the investigation revealed that the two males, one of them armed, stopped Walker on the 2000 block of the avenue and announced a robbery. When Walker went for his off-duty weapon, the gunman shot him three times - once in the chest, once in the stomach, and once in an arm, the captain said.
The officer died a short time later at Hahnemann University Hospital.
Funeral arrangements were pending Monday.
Anyone with information was asked to call 911 or the Homicide Unit at 215-686-3334 or -3335, or to submit a tip at www.phillypolice.com.
The reward includes money from the City of Philadelphia ($20,000), Fraternal Order of Police Lodge 5 ($15,000), Citizens' Crime Commission ($11,000), James J. Binns law firm ($5,000), Model Consulting Co. ($5,000), International Brotherhood of Electrical Workers Local 98 ($5,000), International Association of Fire Fighters Local 22 ($5,000), Jennings Sigmond law firm ($5,000), Metropolitan Regional Council of Carpenters ($5,000), Pennsylvania State Fraternal Order of Police ($5,000), Philadelphia Gas Workers Local 686 ($2,500), First Trust Bank ($2,000), the Retired Police, Fire and Prison Guard Association ($2,000), and the city police Public Affairs Unit ($500).
Contact Joseph Gambardello
at 215-854-2153 or firstname.lastname@example.org.
Inquirer staff writer Robert Moran contributed to this article.