A gunman who fatally shot four people at Tulsa, Oklahoma, medical center targeted a doctor he blamed for the pain he was in after a recent back surgery, police said Thursday. 

Tulsa Police Department Chief Wendell Franklin identified the four people killed in Wednesday’s mass shooting as Dr. Preston Phillips, Dr. Stephanie Husen, Amanda Glenn and William Love. Franklin said the gunman, identified as Michael Louis, recently had surgery and targeted Phillips, who performed the surgery.

“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.”

Franklin said the gunman, who died of an apparently self-inflicted gunshot wound, used two recently purchased firearms, including an AR-15 style rifle, in the Wednesday afternoon shooting at the St. Francis Health System campus.

Wednesday’s attack comes after high-profile shooting massacres in Buffalo, New York, where 10 Black people were killed, and Uvalde, Texas, where 19 children and two teachers were killed in May.