An alert off-duty Atlanta police officer helped catch a teen charged with attempting to rob a gas station.

Surveillance video from the Shell station on Bolton Road taken Wednesday night showed a 15-year-old boy who appears to pull out a gun and point it at the clerk while demanding money, police said.

The teen didn't realize an off-duty police lieutenant witnessed the incident.

Lt. Terry Joyner was driving home around 11:30 p.m. when he saw the teen and some friends come running out of another area business.

He said the behavior made him curious.

"I actually pulled over here and sat and watched them for a few minutes,” Joyner told Channel 2's Amy Napier Viteri.

He said the teen and a 12-year-old girl with him walked across the street to the Shell station.

"As a police officer, I thought, 'Man it looks like this guy is robbing the store,'" Joyner said. "This teenager came in, pointed the gun, demanded money and apparently got scared at that point when he realized this guy was behind bulletproof glass."

By that time Joyner had called for backup. He and another officer tracked the group of teens down to nearby Forest Road.

They recovered the gun, which turned out to be fake, police said.

"If this clerk had a real weapon and he had returned fire in self-defense, both of them could have been killed," Joyner said.

The store manager agreed. He told Viteri off camera this could have been much worse.

"It's dangerous. Dangerous for them and dangerous for others too," he said.

Joyner said real or fake, the act of trying to rob someone is a serious crime.

"If somebody's brazen enough to go in with a toy gun, they would go in with a real gun too. There's no doubt in my mind," he said.

Joyner said the teen was taken into custody and charged with armed robbery.