(TULSA, Okla.) -- A patient gunned down his surgeon and three other people in a mass shooting at a Tulsa, Oklahoma, medical building on Wednesday after blaming the doctor for his pain, according to police.
On May 19, the suspected gunman underwent back surgery. Dr. Preston Phillips -- one of the four people killed in the shooting at the Natalie Building, a five-story medical complex at Saint Francis Hospital -- was the suspect's doctor in that surgery, Tulsa Police Chief Wendell Franklin said at a news conference Thursday.
A letter recovered on the suspect, Michael Louis, made it clear "that he came with the intent to kill Dr. Phillips and anyone who got in his way," Franklin said. "He blamed Dr. Phillips for the ongoing pain following the surgery."
Louis bought an AR-style semi-automatic rifle just hours before the shooting, Franklin said. He bought a 40-caliber pistol from a pawn shop on May 29, police said. Both appeared to be legally purchased, police said.
The three others killed were Dr. Stephanie Husen, office employee Amanda Green and patient William Love, Franklin said.
"They stood in the way" so the suspect "gunned them down," Franklin said.
Louis in the lobby area of Phillips' office from an apparent self-inflicted gunshot wound, police said.
Louis was released from the hospital on May 24, five days after his surgery, Franklin said. After his release, the suspect called several times over several days complaining of pain and wanting additional treatment, the chief said.
On Tuesday, Phillips saw the suspect again for additional treatment, the chief said. On Wednesday, the suspect called the doctor complaining of back pain and wanting additional help, the chief said.
At 4:52 p.m. local time Wednesday, a patient on a video chat with a doctor called 911 after the doctor told her there was shooting, police said. The first 911 call came in at 4:53 p.m. and police arrived at 4:56 p.m., the chief said.
A gunshot heard at 4:58 p.m. was believed to be the suspect's self-inflicted gunshot wound, police said.
Phillips was found dead in an exam room, police said.
As police continued searching the hospital, they said they found a woman hiding under a desk near the suspect's body. The woman said she was unharmed but saw the suspect shoot himself, police said.
Several victims were treated at hospitals for gunshot wounds and later released, officials said.
There's no evidence that Phillips had any concerns about Louis, hospital officials said.
Thirty minutes after the shooting, authorities received a call from a woman who said her husband killed multiple people at Dr. Phillips' office, police said. Louis allegedly contacted his wife before or during the shooting "to let her know what he had done," the chief said.
The chief called this latest mass shooting a "senseless tragedy."
"This is yet another act of violence upon an American city," Franklin said. "We train for instances such as this. And I'm overwhelmed and proud of the men and women, all those that responded."
"Our training led us to take immediate action without hesitation," Franklin said.
Tulsa Mayor G.T. Bynum told reporters, "I spoke with an officer yesterday who is a seasoned veteran of the department. And he was one of the first people to get here. And he said, 'Mayor, it was like the beaches of Normandy out there. Everywhere I looked, to my left and my right, there were officers running towards that building, jumping over bushes, getting around anything in their way between them and that threat so that they could save people.'"
Dr. Ryan Elizabeth Parker, associate chief medical officer at St. Francis, said at Thursday's news conference, "We were just starting to process the grief and emotions that being on the front line of a pandemic had left with us, and now this tragedy. ... Our job is to help and heal, and we are here to do our job, even if it's with broken hearts."
ABC News' Jenna Harrison Esseling and Matt Fuhrman contributed to this report.