The suspect entered the Big Bear Supermarket at South DeKalb Mall in Decatur, in the Atlanta-metro area, where there was an argument with the cashier over a mask. Maddox gave few details about the argument but said it was over a face mask.
The suspect then shot the employee and she died from her injuries, Maddox said at a news conference Monday.
A reserve deputy working security at the store fired at the suspect, who returned fire. Both were injured and taken to local hospitals.
Maddox said the reserve deputy was a 30-year veteran of the force prior to retiring and joining the reserve unit. He is in stable condition.
The Georgia Bureau of Investigation and DeKalb County Police are investigating the incident.