Investigators said the haul from the robbery outside a Wells Fargo bank was more than $300,000, WPVI-TV reported. At least four men appear to have been involved, escaping in a minivan they abandoned nearby.
No arrests had been made late Thursday.
Marnan Harner, who was working in a hairdressing salon across the street, said the guard told her at first he thought the robbers were carrying out a Halloween joke, KYW-TV reported. But Edward Hanko, special agent in charge for the FBI, said it was clearly no joke.
"The timing of such, the armored car arriving at a certain time, would just indicate that it was preplanned and well planned," Hanko said. "One masked subject ordered the guard to the ground while pointing a gun at him and the other subject went for the money."
Investigators said the abandoned van had been reported stolen in August. The robbers apparently escaped in another car.
Duggar sisters unveil Christian dating rules in new book
LGBT community has 'bullied the American people': Bachmann