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.
Ali was scheduled to be arraigned later Saturday. He 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.
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," Donna Alullo, the widow of slain guard Joseph 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 Ali Friday afternoon.
Police believe 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 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.
Loomis does not require guards to wear the vests, which cost $350 to $700, but it encourages their use, spokesman Mark Clark said. The company pays for part of the cost of the vests and offers interest-free loans to employees for the rest.
"One of the things that we're re-evaluating is the vest policy," Clark said. "Our objective is we want people to wear them."
Surveillance tape from the Wachovia bank's security camera and another northeast Philadelphia business showed the robber getting out of his car and putting on gloves before killing William Widmaier, 65, and Alullo, 54, according to police. Both were retired Philadelphia police officers.
Police released cropped still frames from the bank surveillance tape that show a man wearing a yellow baseball cap and bright yellow or orange gloves pointing a handgun toward the front of the ATM.
Widmaier, a city police officer from 1966 to 1989, and Alullo, who served on the force from 1973 to 2000, were assigned to the same district, where they became friends, police Commissioner Sylvester Johnson said.
Widmaier was shot once in the chest, and Alullo was shot three times in the chest and abdomen, police said.
The third guard was grazed by shattered glass as the robber tried to shoot through the armored car. That guard was treated at a hospital for lacerations and released.
Widmaier, married with adult children, was the union shop steward for the Pennsauken, N.J.-based division of Loomis, where the men worked. Alullo was married with three daughters.