A man who shot dead four people at a hospital in the US had been a disgruntled patient suffering back pain post-surgery, police say.
Police Chief Wendell Franklin said the gunman, identified as Michael Louis, had been released from the hospital in Tulsa, Oklahoma, just days earlier on May 24.
Addressing media on Thursday (local time), Police chief Franklin said Louis had targeted the doctor who had treated him, explaining himself in a note he penned before the rampage.
"We 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," he said.
"He blamed Dr Phillips for the ongoing pain following the surgery."
Surgeon Dr Preston Phillips was killed along with Dr Stephanie Husen, a sports medicine specialist, and receptionist Amanda Glenn and patient William Love.
Police Chief Franklin hailed the bravery of the victims.
"They stood in the way. They stood in the way and Louis gunned them down," he said.
Gunman visited victim days before
Louis had met with Dr Phillips as late as Tuesday complaining of back pain and wanting more assistance. He phoned his office on Wednesday for help with his recovery, one of several calls to Dr Phillips' office in recent days.
Police say Louis had purchased an assault-style AR-15 rifle just hours before the shooting from a local gun shop. He also brought a pistol that he purchased at a pawn shop three days earlier.
Louis took his own life after the shooting, police believe.
Several others were injured in the mass shooting however authorities did not specify the number of victims other than it was less than 10.
A bomb threat was also linked to the shooting, police said on Wednesday, but there were no explosives found at the address 75km away.
Dr Cliff Robertson, president and CEO of Saint Francis Health System, said the idea anyone would go after Dr Phillips was "mindblowing", CNN reported.
"Dr Philips was the consummate gentleman," Dr Robertson told media at Tulsa's Heart Hospital.
"The three best people in the entire world that are the most committed to doing what they do every day, and taking care of others, didn't deserve to die this way," he said, also referring to Dr Husen and Ms Glenn.
Mass shootings happening 'anywhere'
The shooting is the latest in a long line of mass shootings to rock the US. It comes just a week after 21 people, including 19 children, were shot dead at a primary school in Uvalde, Texas.
Last month, a teenager livestreamed a mass shooting where he shot dead 10 people in a "racially motivated" attack at a Buffalo supermarket in the state of New York.
The daughter-in-law of a woman at the hospital for a doctor's appointment bemoaned the danger posed to society as a result of the US's gun laws, saying a mass shooting can "happen anywhere".
"You can't even go to a store, you can't even go to school, now you can't go to the doctor?" she told CNN.
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