Richmond County sheriff's Lt. Calvin Chew said there were about six customers in Fifth Third Bank at 2756 Peach Orchard Road when the gunman entered.
Bank employees told officers the man, who had a white towel wrapped around his face, entered the south door and demanded everyone inside hit the floor. He approached a teller with a semiautomatic handgun and demanded she place money in a blue canvas bag he had brought with him.
"She gave him what she could and included a dye pad," Chew said.
The robber fled across Peach Orchard Road where he disappeared behind Southside Monument at 2715 Peach Orchard Road.
Employees told investigators they saw the dye pack explode as he fled.
William Johnson, of Washington, Ga., was driving on Peach Orchard Road and saw the robber dart into the street, then stop in the turn lane and look down at the dye coming out of his bag.
"You just see some idiot running off in traffic with a bag, and this red smoke started coming out of it," Johnson said. "Then it dawned on me."
Johnson immediately went to a local business and asked someone to call 911. He later described the robber as about 6 feet tall, between 200 and 220 pounds, wearing a baggy white T-shirt, baggy blue jeans and black shoes.
Police searched the area but didn't find the robber. Officials later said they think the gunman left in a vehicle because of tire tracks discovered in the vicinity.
No customers or employees were injured, Chew said.
Staff Writer Luke Thompson contributed to this story.