Dr. Preston Phillips, a beloved surgeon at Saint Francis Hospital in Tulsa, Oklahoma, is one of four people who died on Wednesday when a gunman went to the medical building with the intention of killing the doctor. According to The Washington Post, 45-year-old Michael Louis blamed Phillips for the pain he was experiencing after going in for back surgery in May.

Speaking to reporters on Thursday, Tulsa Police Chief Wendell Franklin said Louis bought a semiautomatic rifle and drove to the medical building with the intention of killing Phillips. Franklin said Louis killed the 59-year-old along with another doctor, Stephanie Husen, as well as receptionist Amanda Glenn and William Love, who was accompanying his wife to an appointment.

“They stood in the way and he gunned them down,” Franklin said, according to the Post.

After going in for surgery on May 19, Louis was released less than a week later, police said. He then called several times in the following days, complaining about pain and requesting treatment, according to authorities.

Louis, who met with Phillips on the day before the shooting, called again on Wednesday to express his discomfort and ask for additional aid. Later on Wednesday, Louis legally purchased a .223 caliber semiautomatic rifle and went to the hospital, police said. The gunman, according to Franklin, also brought a .40-caliber pistol he bought from a pawnshop on Sunday.

Louis, who shot and killed himself at the scene after carrying out the attack, carried a letter with him. Police said the letter detailed how the shooter planned to kill “Dr. Phillips and anyone who came in his way,” the Post reports.

According to the Daily Beast, Saint Francis Health System released a statement on Wednesday, saying it’s “grieving the loss of four members of our family.”

Phillips is remembered by his colleagues, friends, family and patients as a difference maker. Sandy Thompson, a friend of the surgeon, told CNN that he was “a wonderful, gentle, kind person with an infectious smile and kind word for everyone.”

Thompson said Phillips was a “true gentleman” who never failed “to look you in the eye and give you a bright, genuine smile and warm greeting.”

“He made the world a better place and Preston’s presence will be deeply missed,” she told CNN.

The Tulsa doctor was a 1990 Harvard Medical School graduate. He most recently worked as an orthopedic surgeon with an interest in spinal surgery.

Ruth Folly, a surgical technician who joined Phillips on several medical missions to Africa, said the doctor traveled yearly with a nonprofit organization known as Light in the World Development Foundation. The group, founded by Phillips’ colleague and Folly’s father, Dr. Komi Folly, provided surgical services to those in underserved areas.

As part of their mission, Folly’s father and Phillips had been traveling to Togo in West Africa since 2016.

“You always call me a son and I call you father,” Folly wrote on Facebook. “You told me last week at work during lunchtime to not stop this project in Togo in case something happens to you. I did not know that you were giving me the last advice for our journey. I am so sad to see you go without a goodbye. Your work will continue. You will be missed.”

According to the Gun Violence Archive, Wednesday’s shooting is the country’s 233rd mass shooting of 2022. The tragedy comes shortly after a gunman killed 19 students and two teachers at Robb Elementary School in Uvalde, Texas.

“I knew it was an active shooter but I didn’t know the extent. It’s very sad. We just keep having them, every day, every year, until we decide that we’ve had enough,” Debra Proctor, a nurse at Saint Francis Hospital, told The Daily Beast. “In my mind, it’s like a massive car collision. Then I thought maybe it’s a fire. But then you realize the active shooter is right next door.”