A man who fatally shot four people at an Oklahoma medical building had gone there to kill a doctor he blamed for back pain he felt after surgery, authorities say.
Michael Louis entered a building on Tulsa's St Francis Health System campus with a semi-automatic weapon on Wednesday and opened fire at anyone he encountered, Tulsa Police Chief Wendell Franklin said at a news briefing.
Two doctors, a receptionist and a patient were killed.
Dr Preston Phillips, 59, the orthopedic surgeon who treated the gunman, was killed along with Dr Stephanie Husen, a 48-year-old sports medicine specialist.
The suspect "came in with the intent to kill Dr Phillips and anyone who came in his way", Franklin said. Authorities found a letter on the gunman that made it clear the attack was targeted.
Authorities named two other fatalities: Amanda Glenn, a receptionist, and William Love, a patient.
"They stood in the way and (the suspect) gunned them down," Franklin said.
The gunman, who police said lived in Muskogee, about 80 kilometres from Tulsa, had been released from the hospital on May 24 after back surgery, the police chief said.
Afterwards, the man called several times complaining of pain.
The shooting comes on the heels of two other mass killings that have stunned Americans and reopened a long-standing debate over gun laws and mental health in the gun violence plaguing the United States.
"Enough is enough. This must stop. Hospitals are pillars of our communities," Chip Kahn, chief executive of the Federation of American Hospitals, said in a statement.
The gunman bought an assault-style rifle at a local gun shop earlier on Wednesday, authorities said. He also bought a pistol at a pawn shop three days earlier.
He parked on the second floor of the garage attached to the five-storey Natalie medical office building.
He entered through the second-floor entryway and worked his way into the building, Franklin said.
Police arrived at the scene three minutes after receiving a call just before 5pm.
Officers followed the sound of gunfire to the second floor, and made contact with the victims and the suspect five minutes later, the chief said.
Officers at the scene said they heard a gunshot five minutes later, which Franklin said was the gunman taking his own life.