A gunman who killed four people at a US medical facility has been identified as a patient who had recently been operated on by one of the doctors he shot dead on Wednesday.

The mass shooter, identified as Michael Louis, targeted surgeon Dr Preston Phillips – who performed back surgery on him days earlier – at the Tulsa medical building, authorities said on Thursday.

Tulsa Police Chief Wendell Franklin told reporters that Louis, 45, legally bought a semiautomatic AR-15-style rifle from a local gun store about one hour before the shooting, and purchased a semiautomatic handgun on May 29 from a local pawn shop.

Michael Louis killed four people, including Dr Preston Phillips. Photo / Tulsa Police Department

At about 5pm on Wednesday local time, he walked into the orthopaedic office on the second floor of the Natalie Medical Building at St Francis Hospital in Tulsa, in the state of Oklahoma, and opened fire.

Louis killed four people and then himself, according to police.

Two of the four victims were doctors at the hospital, including Phillips.

Investigators later found a note on the scene that authorities said "made it clear" that the gunman intended to "kill Dr Phillips and anyone who got in his way."

"He blamed Dr Phillips for the ongoing pain following the surgery," Franklin said during a press briefing.

The other victims were Dr Stephanie Husen, the facility's receptionist Amanda Green and William Love who was a patient.

"They stood in the way and Louis gunned them down," Franklin said.

"He began firing at anyone who was in his way."

Killer's calls complaining of back pain

Louis, of Muskogee about 80km southeast of Tulsa, was admitted into the hospital on May 19 for the procedure performed by Phillips. He was released on May 24, Chief Franklin said.

Louis proceeded to call several times to complain of pain and sought additional treatment. He then saw Phillips on Tuesday and called the doctor's office again on Wednesday seeking additional help.

Tulsa police and firefighters respond to the shooting. Photo / AP

A page on Saint Francis' website identified Phillips as a board-certified orthopaedic surgeon specialising in spinal surgery and joint reconstruction, the New York Post reported.

The Saint Francis Health System said late on Wednesday it was grieving the loss of "four members of our family".

"As a faith-based organisation, the only recourse we have at this moment is to pray while we navigate this tragedy.

All appointments at the orthopaedic clinic in the Natalie Building will be "closed until further notice". Saint Francis officials said.

The attack marked the 233rd mass shooting in the US this year, according to the Gun Violence Archive, which defines such incidents as at least four people being shot or killed excluding the shooter.

The tragedy occurred just one week after an 18-year-old gunman slaughtered 19 students and two teachers on May 24 at Robb Elementary School in Uvalde, Texas.

Flags throughout Oklahoma were ordered to be flown at half mast for four days in the aftermath of the mass shooting, officials said.

Police recovered dozens of spent shell casings from the shooting scene.

- Megan Palin, news.com.au