Police believe they live in the neighborhood and hope someone will recognize them from the video.
Also Tuesday, the reward for information leading to the arrest and conviction of Walker's killer climbed to $118,000. That includes money from the city, Fraternal Order of Police Lodge 5, and a number of unions, law firms, and businesses.
"Hopefully, it's an incentive to someone to give us a call," Police Commissioner Charles H. Ramsey said. "But with or without that, we would hope that people would step up and help us get two violent people off the street. These guys will rob again, they will kill again, if they're not taken off the streets."
Funeral arrangements for Walker, 40, are pending, Ramsey said Tuesday.
Walker, a 19-year veteran of the force, crossed paths with the robbers shortly before 6 a.m. as he walked to a bus stop.
Police said investigators have received videos from a number of sources in the neighborhood, including businesses and residents. They are following up on tips they have received and have interviewed at least one witness.
Ramsey said the community has a "collective interest" in making sure the men were apprehended.
"They're vicious, violent killers," Ramsey said. "It's predator and prey. . . . Until these folks are off the streets and in jail for a long period of time, it's not going to stop. Today, it's somebody else. Tomorrow, it could be you."
On Monday, investigators released surveillance video that showed Walker walking west on Cecil B. Moore at 19th about 5:55 a.m. to catch a bus home after completing his shift at the nearby 22d District. Walker, who was wearing shorts and a baseball cap and carrying a backpack, appeared to notice the men before the robbery, and could be seen glancing over his shoulder.
A second camera view shows Walker walking into the street, around a car, and back to the sidewalk, apparently wary of the men. The same camera view then catches the men coming up on the other side of the street, then crossing the street and following Walker.
A third man, whom police have interviewed, also crossed the street and followed the pair.
Capt. James Clark of the Homicide Unit said the investigation revealed that the two males, one of them armed, stopped Walker on the 2000 block of Cecil B. Moore and said they were going to rob him. When Walker went for his off-duty weapon, the gunman shot him three times - in the chest, in the stomach, and in an arm.
Walker died a short time later at Hahnemann University Hospital.
Authorities ask that anyone with information call 911 or the Homicide Unit at 215-686-3334 or -3335, or submit a tip at www.phillypolice.com.
Contact Allison Steele at 215-854-2641 or firstname.lastname@example.org.