Tennessee ? A mentally ill convenience store operator took a revolver with him to look for the doctor he believed implanted a tracking device in his body during an appendectomy in 2001. Told the doctor wasn’t at the hospital, he went to a nearby parking lot and opened fire on three hospital workers he apparently didn’t know, killing one of them.

Gunman Abdo Ibssa entered a medical tower near Parkwest Medical Center before Monday’s attack and asked for the doctor who performed the appendectomy, Knoxville Police Chief Sterling Owen IV said. Unable to find the doctor, Ibssa went to another area where patients are discharged and opened fire on the women as they walked out of the building.

Ibssa, who police believe had been skipping doses of medication for psychotic behavior, killed himself to end the shooting, which occurred a day before his 39th birthday.

“There was less than 5 seconds from the time of the first shot until the last shot,” Owen said at a news conference Tuesday.