District Attorney Jimmy Dunn said a former police officer in Bean Station was taken into custody shortly after the shooting and robbery Thursday at the Down Home Pharmacy.
Dunn said the former officer, 37-year-old Jason B. Holt, took "a large quantity of pills" during the late-morning robbery.
The Knoxville News Sentinel reports that the pharmacy owner and the customer died at the scene.
Dunn said the two employees were flown by helicopter to the University of Tennessee Medical Center, where they were in stable condition.
Holt was apprehended by local police and sheriff's officers. Tennessee Bureau of Investigation agents were summoned to the scene to assist.
The town's population is less than 3,000.