The thieves were bundled up for the cold weather in heavy clothing, the New York Times reported. They escaped in an SUV after the robbery.
The robbery occurred about noon outside an HSBC branch in the Elmhurst neighborhood in Queens, the FBI said. Investigators said the guard was sprayed with Mace and threatened with a gun.
Security cameras showed one of the thieves holding a gun on the guard, the FBI said.
The FBI only said a "significant amount of cash" was taken. But a source confirmed to the Times that the haul was $300,000.
GardaWorld, the company that operates the truck, is offering a $50,000 reward for information leading to arrests and to the stolen money.