A gunman who killed four people at an Oklahoma hospital targeted the doctor who performed his back surgery, Tulsa's police chief has said.
The man who carried out the shooting at the St Francis Hospital campus in Tulsa on Wednesday was identified by police as Michael Louis.
The gunman blamed the doctor – Dr Preston Phillips – who carried out his back surgery for the pain he was in. Dr Phillips was one of those killed.
Another doctor and two others were murdered after the gunman opened fire with a handgun and an AR-15-style weapon – the same type of semi-automatic gun used in the recent Texas school shooting.
Louis had purchased one gun an hour before the shooting and the other three days before. Both are believed to have been bought legally, said Tulsa police chief Wendell Franklin.
He killed himself after a shooting spree that lasted about five minutes.
The details come ahead of a President Joe Biden speech on gun laws later on Thursday. He wants to press Congress "to pass common sense laws to combat the epidemic of gun violence that is taking lives every day", the White House said.
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Police added that Louis had parked on the second storey, where Dr Phillips' office was located, and shot "indiscriminately" at whoever got in the way.
Chief Franklin said Louis had a letter on him that "made it clear that he came in with the intent to kill Dr Phillips and anyone who got in his way".
The other doctor killed was orthopaedic doctor Dr Stephanie Husen, and the others who died were receptionist Amanda Green and patient William Love.
Mr Love had held a door closed to allow someone else to escape when he was shot, while one of the other victims was holding a door open to allow someone else to run out.
Louis had been to an appointment the day before and Dr Phillips had been attempting to help him with his pain after his surgery on 19 May.
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His wife called the police as Louis had contacted her either before the shooting or while it was taking place to let her know what he had done.
There was also a bomb threat at his house, but police are still figuring out the details.
A spokesman for St Francis said: "Dr Phillips was a consummate gentleman, he is a man we should all strive to emulate.
"The fact some individual would go after Dr Phillips is mind-blowing, he's one of those folks that his clinic isn't always on time because he will spend every minute with patients they need.
"Not only is it a shock it's the ultimate loss for St Francis."
Some others were injured in the shooting but they were treated and discharged the same day.