A man who blamed his surgeon for ongoing pain after a recent back surgery bought an AR-style rifle hours before opening fire at a Tulsa medical office, killing the surgeon and three other people before fatally shooting himself, police said Thursday.

Tulsa Police Chief Wendell Franklin says the gunman had recently undergone back surgery and had called a clinic repeatedly complaining of pain.

Franklin says the doctor who performed the surgery, Dr. Preston Phillips, was killed Wednesday, along with another doctor, a receptionist and a patient.

“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.”

Dr. Cliff Robertson, president and CEO of Saint Francis Health System, called Phillips “the consummate gentleman" and “a man that we should all strive to emulate.” He said the three employees who were killed were “the three best people in the entire world” and that they “didn’t deserve to die this way.”

Authorities said the gunman carried a rifle and handgun during the shooting at the medical building on a hospital campus, the latest in a series of deadly mass shootings across the country in recent weeks.

Wednesday's shooting on the campus of Saint Francis Health System happened the same week that families in Uvalde, Texas, began burying the dead from the deadliest school shooting in nearly a decade.

The victims and gunman in Tulsa were found on the second floor of a medical office where an orthopedic clinic is located, police said. The shooter, whose name has not been released, died from an apparent self-inflicted gunshot wound, Tulsa Police Department Deputy Chief Eric Dalgleish said.

The hospital said in a statement that it was “grieving the loss of four members of our community” but did not immediately identify the dead.

All appointments at the orthopedic clinic were canceled through Friday.

The spate of recent gun violence across the country, including the killing of 19 children and two teachers at Robb Elementary School in Uvalde by an 18-year-old gunman carrying an AR-style semi-automatic rifle, has led to Democratic leaders amplifying their calls for greater restrictions on guns, while Republicans are emphasizing more security at schools.

The divide mirrors a partisan split that has stymied action in Congress and many state capitols over how best to respond to a record-high number of gun-related deaths in the U.S.

Authorities investigating the Tulsa shooting executed a search warrant at a home in Muskogee, about 45 miles (70 kilometers) southeast of Tulsa, in connection with the investigation, police said.

“It appears both weapons at one point or another were fired on the scene," Dalgleish said. “The officers who arrived were hearing shots in the building, and that's what led them to the second floor."

Police responded to the call about three minutes after dispatchers received the report at 4:52 p.m. and made contact with the gunman at 5:01 p.m., Dalgleish said.

“I was very happy with what we know so far regarding the response of our officers," Dalgleish said.

The length of time it took police officers in Uvalde to engage the gunman during last week's deadly school shooting in Texas has become a key focus of that investigation. Officers waited over an hour to breach the classroom where the gunman attacked.

Tulsa Police Capt. Richard Meulenberg also said multiple people were wounded and that the medical complex was a “catastrophic scene.” The exact number of wounded was not immediately available.

St. Francis Health System locked down its campus Wednesday afternoon because of the situation at the Natalie Medical Building, which houses outpatient clinics and other medical offices.

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

Bynum added: “Right now, my thoughts are with the victims. If we want to have a policy discussion, that is something to be had in the future, but not tonight."

Philip Tankersley, 27, was leaving his father’s room at nearby Saint Francis Hospital around 5 p.m., when hospital staff said there was an active shooter in the building across the street, locked the doors and warned them to stay away from the windows.

Tankersley said he and his mother sheltered in his father’s hospital room for more than an hour, trying to learn scraps of information from the TV news and passing nurses. He said they heard “code silver” and “level 1 trauma” announced on the hospital speakers and wondered if they were safe in the room.

“I wasn’t particularly worried because the two people that I need to look out for were in that same room as me,” he said. “But it was definitely a ‘this is happening here’ moment.”

The shooting Wednesday also comes just more than two weeks after shooting at a Buffalo supermarket by a white man who is accused of killing 10 Black people in a racist attack. The recent Memorial Day weekend saw multiple mass shootings nationwide, including at an outdoor festival in Taft, Oklahoma, 45 miles from Tulsa, even as single-death incidents accounted for most gun fatalities.

Since January, there have been 12 shootings where four or more people have been killed, according to The Associated Press/USA TODAY/Northeastern University mass killing database. Those shootings have left 76 dead, including 31 adults and children in Buffalo and Texas, the database says. The death toll does not include the suspects in the shootings.