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

Mustafa Ali, 36, of Philadelphia, then shot the two retired Philadelphia police officers as they serviced the cash machine, 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 a supervisor and is not authorized to comment publicly.

Mr. Ali was arrested on an unrelated charge Friday afternoon and was later charged with two counts of murder, robbery, a firearms charge, and other charges after giving a statement.

Mr. Ali previously served seven years in federal prison for bank robbery, the detective said. It was not clear if he had an attorney.

“If he is the suspect that killed my husband, he hurt his own family too,” the widow of one the slain guards, Donna Alullo, said after he was brought in for questioning. “We hope that it is the suspect … so that he is off the street and doesn’t do it to anyone else.”

The slayings occurred Thursday while the guards were servicing an ATM, police said.

The robber approached the armored car from behind Thursday morning outside a bank in northeast Philadelphia. He shot one guard in the chest, then went around the vehicle and shot the second guard as he tried to unholster his gun, police said. He shot toward a third guard inside the armored car, picked up a bag of deposits and fled.

Investigators believe he may have grabbed deposits — likely a mix of checks and cash — but the detective said Saturday that authorities don’t believe he got away with much.

Authorities had spent Friday fielding tips about the suspect and his getaway car, an Acura TL Type-S. Police towed a car matching that description from the apartment complex where they found Mr. Ali Friday afternoon.

Police believe Mr. Ali was preparing to leave the area when he was arrested.

Police have recovered the gun used in the shootings, a 9 mm semi-automatic, which Mr. Ali had tossed near a community college, the detective said.

As authorities searched for the robber, officials with the company that employed the guards, Loomis, were in the Philadelphia area meeting with employees and the guards’ families. The company has about 190 total employees in the Philadelphia area, about half of them guards.

Neither of the slain guards was wearing a bulletproof vest.