Tulsa, Oklahoma – The man accused of killing four people at a Tulsa hospital on Wednesday blamed his surgeon for his continued back pain, according to police.

Officials identified the gunman on Thursday as 45-year-old Michael Louis, who had undergone back surgery at the hospital on May 19, Tulsa Police Chief Wendell Franklin announced at a press conference with the Tulsa mayor and hospital officials.

"We also have 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," Franklin said. "He blamed Dr. Phillips for the ongoing pain following the surgery."

Louis opened fire on the second floor of a medical building on the campus of Saint Francis Hospital just before 5 PM local time on Wednesday. He was dead from an apparent self-inflicted gunshot wound by the time police arrived minutes later.

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Louis was allegedly hunting Preston Phillips, who had performed his back surgery on May 19, according to a note he left with relatives, police said at a press conference. In the days before the shooting, Louis had reportedly been complaining about back pain.

"Louis was in pain," Franklin said. "Louis expressed that he was in pain and was not getting relief."

The four victims were identified as Phillips, Stephanie Husen, another doctor, receptionist Amanda Green and William Love, a patient.

Saint Francis’ trauma team rushed to try to save Love’s life, but were unsuccessful.

"We so wanted to be able to utilize our skills and training to save these precious lives," Ryan Parker, chief of emergency medicine at Saint Francis, said Thursday. "To the family of Mr. Love, I’m so sorry we couldn’t save you."

Love held a door open to allow another patient to escape before he was shot, officials said.

Louis had purchased an AR-15-style rifle hours before the shooting, according to officials. Days earlier, he also bought a .45 caliber Smith and Wesson handgun from a local pawn shop.

"The shooter is dead. The threat that he brought to this campus, that has been ended," Tulsa Mayor GT Bynum said on Thursday.

"But the days ahead of us and the weeks ahead of us, it is so important for this community to show the team at Saint Francis how much we love you. And I want to thank you all for coming to work today."