PHILADELPHIA — A convicted bank robber charged with killing two armored car guards had spotted the vehicle on the road and followed it to the ATM where he opened fire, a homicide detective said Saturday.

Mustafa Ali, 36, of Philadelphia, executed the two retired Philadelphia police officers without a word, police said.

"He follows the truck, he sees an opportunity and he takes it," said the detective, who worked on the case but requested anonymity because he is not authorized to comment publicly.

Ali owned the dark Acura used in the heist, although the FBI had not been able to glean the license plate number from the surveillance film, the detective said. Ali instead became a suspect based on tips called in from the public.

Commissioner Sylvester Johnson, whose department has struggled to solve murder cases amid a 'stop snitching' culture, thanked the community for stepping up in this case.

"Within an hour, we started getting information from citizens," Johnson said at a Saturday afternoon news conference. "(Ali) will never walk the streets, hopefully, again in his lifetime."

Ali, who is married and employed, previously served seven years in federal prison for armed bank robbery under the name Shawn Steele, according to federal court records.

A 1993 indictment charged three men, including the 21-year-old Steele, with stealing a combined $25,000 in eight Philadelphia bank robberies in 1992. His supervised release ended in 2004.

Ali was arraigned shortly after 9 p.m. Saturday and was held without bail. He was represented by a public defender, and a preliminary hearing was set for Oct. 10. He was arrested on an unrelated warrant Friday and later charged with two counts of murder, robbery, a firearms charge and other crimes after giving a statement.

Police recovered the gun, a 9 mm semiautomatic, near a northeast Philadelphia community college after Ali told them where he had tossed it, the detective said.

The victims, William Widmaier, 65, and Joseph Alullo, 54, were friends who worked together on the police force. They went to work for Loomis, the armored-car company, after they retired — Widmaier in 1989 and Alullo in 2000.

Loomis does not require its guards to wear bulletproof vests, although the company said it would now review that policy. The slain men did not have them on.

Surveillance tapes from the Wachovia bank security camera and another nearby business show the robber getting out of his car and putting on gloves before firing.