Tulsa gunman bought AR-15-style rifle hours before using it to kill his former doctor, 3 others: Report.

Tulsa gunman bought AR-15-style rifle hours before using it to kill his former doctor, 3 others: Report.

A gunman who killed four people at a Tulsa hospital on Wednesday targeted a doctor who had treated him and bought the AR-15-style rifle used in the shooting on the day of the attack, police said.

Officials said the gunman was armed with a rifle, which he had purchased at 2 p.m. on Wednesday, and a handgun when he went to St. Francis Hospital shortly before 5 p.m.

He opened fire inside the Natalie Building, killing doctors Preston Phillips and Stephanie Husen, receptionist Amanda Glenn and patient William Love, police said at a news conference Thursday.

Tulsa Police Chief Wendell Franklin said that the shooter was a patient of Phillips and underwent back surgery on May 19.

The shooter was released on May 24 and repeatedly complained of being in pain in the days afterward. On May 31, the shooter received additional treatment at the hospital but still complained of pain, Franklin said.

The chief said the shooter “blamed” Phillips for his pain.

Hours before the killings, the shooter purchased an AR-15 style rifle from a gun store. Days before that he had bought a semi-automatic pistol from a pawn shop, according to Franklin.

The chief said the shooter had a letter with him “which made it clear that he came in with the intent to kill Dr. Phillips and anyone who got in his way.”

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

The shooting began around 4:52 p.m. on the second floor of the five-story building. A doctor was on a video call with someone and told the person to call 911 to report shots fired, the chief said. Police received the first report at 4:53 and were on the scene three minutes later. 

Franklin said the shooter parked in the parking garage and entered through a second-floor door. As soon as he walked in “he began firing at anyone that was in his way,” the chief said. 

The shooter died from an apparent self-inflicted gunshot wound as officers were making their way to the second floor. Either before the chaos or during, the shooter called his wife who alerted authorities, according to Franklin.

Franklin said Phillips was found dead in an examination room. Other people were wounded but a hospital official did not say how many or if it was from gunshots. Those who were wounded have been treated and released, the hospital official said.

The shooting follows killings in Buffalo, New YorkUvalde, Texas; and New Orleans. A span of violence also hit several U.S cities over Memorial Day weekend leaving at leas 156 people dead and 412 injured from 5 p.m. ET Friday to 5 a.m. Tuesday, according to a preliminary count by The Gun Violence Archive.

This report as originally published on NBC News.