The gunman who shot dead four people at a hospital in the US state of Oklahoma killed the surgeon whom he blamed for pain after back surgery, police said Thursday.

The suspect, identified as Michael Louis, stormed into the Saint Francis hospital in the city of Tulsa on Wednesday with a rifle and a handgun, in the latest mass shooting in the United States in recent weeks.

Louis had recently been operated on by surgeon Preston Phillips at the hospital and had called the clinic to complain of continuing back pain, Tulsa Police Chief Wendell Franklin said during a press conference.

Police found “a letter on the suspect, which made it clear that he came in with the intent to kill Dr. Phillips and anyone who got in his way,” Franklin said.

“He blamed Dr. Phillips for the ongoing pain following the surgery.”

As well as Phillips, the other victims included another physician, a receptionist and a patient.

“They stood in the way and Lewis gunned them down,” Franklin said, adding Louis then shot himself.

The suspect purchased an semi-automatic gun at a local gun shop shortly before the shooting, police said.

The killings come as families in Texas bury their dead after a school shooting left 19 young children and two teachers dead last week.

The tragedies have prompted calls for tighter gun control legislation but US lawmakers have failed to pass significant new laws despite years of worsening gun violence.

According to the Gun Violence Archive, there have been 233 mass shootings this year in the United States. It defines mass shootings as when four or more people are injured or killed in a single event, not including the shooter.