It happened just after midnight Monday at the 7-Eleven on W. Seventh Street near Olive Street.
Investigators say one of the clerks was at the register when the suspect came into the store yelling at him to open the register and give him money. For some reason, the clerk could not get the register open and called for the other clerk to come help.
When neither employee could open it, the suspect left empty handed. Several witnesses saw him running, gun in hand, along Seventh Street, then south on Grand Avenue.
Witnesses also described a second man who may have served as a lookout during the robbery.
"We want everyone to look closely at the video and consider the suspect's clothing description," Lt. Paul Vernon said in a news release. "It's distinct enough that it might be familiar to someone who knows this robber. Just put yourself in this clerk's place and imagine how scary this must have been."
Police say the suspect was wearing a black night-watch cap, a brown and white striped shirt with orange long sleeves from a shirt underneath. He also wore gloves, blue jeans and a gold watch with a black wrist band.
He is described as African American, between 20 and 30 years old, 5 feet 10 inches with a 3-to 4-day growth of beard.
Anyone with information is urged to call (800) 222-8477.