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TULSA, Okla. (NewsNation) — A man who blamed his surgeon for ongoing pain after back surgery walked inside a Tulsa hospital Wednesday and opened fire, killing two doctors, a receptionist and a patient, police said. He then fatally turned the gun on himself.

Tulsa Police Chief Wendell Franklin said the gunman, identified as Michael Louis, 45, of Muskogee, Oklahoma, had undergone back surgery May 19 and afterward called the clinic several times complaining of pain. Police say Louis had an appointment with Dr. Preston Phillips on May 31, but he still seemed dissatisfied with his treatment when he called the office again Wednesday.

According to Tulsa police, Louis used a .223 caliber semi-automatic rifle and a .40 caliber pistol in the attack on the Saint Francis Health System campus. A total of 37 shell casings were recovered from the scene along with a note found on Louis explaining his intent to kill his doctor and anyone who got in his way.

“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.”

The Tulsa Police Department was alerted of the hospital’s active shooter situation at 4:52 p.m. Wednesday by someone who was not on scene after a doctor told her to do so. Officials say authorities responded in minutes, arriving at 4:56 p.m. running into danger as shots were still being fired and making their presence known. At 4:58 p.m., police say the gunman took his own life.

At around 5:24 p.m., police received a call from a sheriff’s office dispatcher who said a woman on the line called saying her husband had killed several people at the doctor’s office.

Police described the scene inside the hospital to a local television station as “catastrophic.” They say the shooting was “fairly limited” to one section of the hospital’s second floor with part of it happening in an orthopedic center.

Tulsa Police identified the victims killed in the shooting as Dr. Preston Phillips, Dr. Stephanie Husen, Amanda Glenn and William Love.

Dr. Preston Phillips

Phillips, 59, worked as a board-certified orthopedic surgeon with specialties in spinal surgery and joint reconstruction.

The Saint Francis Health System CEO called Phillips’ death “the ultimate loss for Saint Francis and for Tulsa.”

Phillips was a 1990 graduate of Harvard Medical School with fellowships completed at Beth Israel Deaconess Medical Center in Boston. He earned advanced degrees in organic chemistry and pharmacology, as well as theology from Emory University.

Amanda Glenn

Amanda Glenn, 40, is described by her family as a devoted wife and mother, loving sister and daughter.

Known to have a true servant’s heart, those who knew Amanda said she always put others above herself.

Amanda had worked in the medical field for more than 18 years. She was happiest spending time with her husband and two sons.

William Love

Love, 73, was a retired Army first sergeant with 27 years of service. He served one tour in Vietnam.

Love’s family reported that at the time of the hospital shooting, he heard the gunshots and knew his wife would not be able to escape the building on her own. They say he sacrificed his own life for hers. The couple had been married since 1967.

Dr. Stephanie Husen

Husen, 48, worked as a sports and internal medicine specialist with the Warren Clinic in Tulsa.

She graduated from the Oklahoma State University Center for Health Services medical school in 2000 and completed her residency at Greenville Memorial Hospital in South Carolina.

Husen was a member of the Chi Omega Sorority. Her chapter shared a message, calling Husen “an incredible woman.”

The wing of Saint Francis where the shooting happened has been closed following the attack.

Gannon Gill, a physician’s assistant, told the New York Times he guided patients through a maze of exam rooms and hallways away from the gunfire. He eventually made his way to a nearby parking lot, where he ran into a patient.

That man told Gill he and his wife ran into the gunman during the attack, and the shooter told the couple to leave because he was not there for them.

“You see this stuff on television or the news,” Gill said to the Times, “but you don’t think it’s ever going to happen in your workplace.”

Dr. Cliff Robertson, president and CEO of Saint Francis Health System, is asking for prayers.

“We’re an organization that believes in the power of prayer and there is nothing more this community could do for us than to pray for the families and the loved ones and the victims of this senseless act,” Robertson said.

Robertson and Tulsa Mayor G.T. Bynum both praised first responders for their quick response, which police said was just minutes from when the 911 call came in to dispatchers.

“I also want to express our community’s profound gratitude for the broad range of first responders who did not hesitate today to respond to this act of violence,” Bynum said.

Oklahoma Gov. Kevin Stitt called the shooting a “senseless act of violence and hatred” and said on Twitter that he and his wife were “praying” for the victims and their families.

“I am grateful for the quick and brave actions of the Tulsa Police Department and other first responders who did their best to contain a terrible situation,” Stitt wrote. “I have offered Mayor G.T. Bynum any state resources that may be needed, and I ask all Oklahomans to come together in support of the Saint Francis Health System community and to grieve with those whose lives have been forever changed.”

Saint Francis Hospital is the largest hospital system in Tulsa, according to retired Tulsa police Officer Sean Larkin. Robertson said it employs more than 10,000 people.

“This campus is sacred ground for our community,” Bynum said. “For decades, this campus has been a place where heroes come to work every day to save the lives of people in our community.”

The Tulsa Community Foundation has a Saint Francis Employee Emergency Fund set up to help members of the Saint Francis family. Those who feel called to help, can visit this link.

This is the 233rd mass shooting of the year, including a massacre at an elementary school in Uvalde, Texas, that killed 19 students and two teachers, and a racially motivated attack at a supermarket in Buffalo, New York, that killed 10.

The Associated Press contributed to this report.