Garrison, 32, was responding to a complaint that Pao was using the Five Guys Burgers and Fries restroom to bathe himself, and he escorted Pao to the back of his patrol car.
In the back of the car, Pao, who was not handcuffed, reached into a bag and a struggle ensued, according to Georgia Bureau of Investigation Agent-in-Charge Pat Morgan.
During the struggle, Garrison shot Pao in the chest, and Richmond County Deputy Coroner Johnny McDonald pronounced him dead at the scene about 6:20 p.m.
A semiautomatic handgun was found concealed under a shirt between Pao's legs, Morgan said.
"It appeared that he'd removed (the gun) from the bag it was in," Morgan said. "We could tell there had been a struggle in the back seat."
Richmond County Sheriff Ronnie Strength identified Garrison but referred all questions to the GBI, which is handling the investigation.
Garrison received his police officer certification April 20, according to Ryan Powell, the director of operations for Georgia Peace Officer Standards and Training. He put himself through the academy and this was his first police job, Powell said.
Once the investigation is finished, the GBI will turn the results over to the Richmond County District Attorney's office for review, Morgan said.
Little is known about Pao, who law enforcement said Monday was homeless.
In the only previous incident Maryland court records reveal, 6-foot, 185-pound Pao was cited in Rockville in March for failure to obey a traffic signal, but the charge was dismissed in May.
"He had somehow ended up down in our area; we don't know how at this point," Morgan said.
Five Guys personnel had contacted law enforcement previously about Pao and were instructed to call the next time he came in, the agent said.
A Five Guys manager said Tuesday he couldn't speak about the incident.
"It all happened outside and nobody knows anything," he said.
The shooting is the sixth -- the fifth fatal one -- involving a Richmond County officer since 2008.