Investigators say the robber somehow came through the ceiling of the Shoe Station on Dawson Road.
He hit security guard Freddie Green in the head with the butt of a shotgun. The guard took the gunman to the office where manager Ta-Tanisha Turner was counting money.
The robber took the cash, the security guard's .38-caliber revolver, and Ms. Turner's car keys and cell phone.
He tied the pair up with plastic bags and left the building by the front door. The guard was treated for his injury.
Firemen surveyed the roof, but they did not find a hole.
Police are looking for a black man, 5-11 to 6-feet 2-inches tall, and 250 to 275 pounds.
He was wearing a black ski mask, maroon T-shirt, and green pants.
If you have any helpful information, call crimestoppers at 436-TIPS.