A gunman who killed his surgeon and three other people at a Tulsa medical office Wednesday blamed the doctor for his continuing pain after a recent back operation and bought an AR-style rifle just hours before the rampage, 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, died, along with Dr. Stephanie Husen, receptionist Amanda Glenn and medical center visitor 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.

Police have received reports that 73-year-old Love, who was accompanying a patient to the doctor’s office, held a door shut in hopes of allowing others to flee from the gunman through another door, Franklin said.

Louis carried a letter that said he was targeting Phillips, Franklin said. The letter “made it clear that he came in with the intent to kill Dr. Phillips and anyone who got in his way,” Franklin said. 

He said Louis called the doctor’s office “several times over several days” reporting he was still in pain and saw Phillips on Tuesday for “additional treatment.” Louis called the office again Wednesday “complaining of back pain and wanting additional assistance,” he said.

Louis bought an AR-style semi-automatic rifle on the afternoon of the shooting and a handgun on Sunday, the police chief said.

Police responded to the call about three minutes after dispatchers received the report at 4:52 p.m. and made contact with the gunman at 5:01 p.m., authorities said Wednesday.