A man who blamed his surgeon for continuing pain after a recent back operation bought an AR-style rifle and opened fire hours later at a Tulsa medical office , killing the doctor and three other people in an attack that ended with him taking his own life, police said Thursday.

The gunman called the clinic repeatedly complaining of pain and specifically targeted the doctor who performed the surgery, Tulsa Police Chief Wendell Franklin said.

That physician, Dr. Preston Phillips, was killed Wednesday, along with Dr. Stephanie Husen, receptionist Amanda Glenn and patient William Love, police said. The attack occurred on the campus of Saint Francis Health System in Tulsa. The chief identified the shooter as Michael Louis, 45, of Muskogee, Oklahoma.

It was the latest in a series of mass shootings in United States including the deadly school shooting in Uvalde, Texas, and an attack on a supermarket in Buffalo, New York.