Shooter Michael Louis, who police said killed himself after the rampage in Tulsa, had just earlier that afternoon legally purchased one of the firearms used in the slaughter, an AR-15 style rifle, city Police Chief Wendell Franklin said during a news conference.
Louis entered a physicians' office building at Tulsa's Saint Francis Hospital campus late Wednesday afternoon and shot dead his erstwhile surgeon, Dr. Preston Phillips, and three other people before fatally shooting himself, said Franklin.
The other three victims at the Natalie Medical Building were Dr. Stephanie Husen, receptionist Amanda Glenn and patient William Love, authorities said.
"He blamed Dr. Phillips for the ongoing pain following the surgery," Franklin said, later calling the letter's contents "a clear motive."
The others, Franklin said, just "stood in the way, and Louis gunned them down."
An unspecified number of other people were injured in the shooting on the building's second floor, officials said at Thursday's news conference, declining to say whether the injuries were caused by gunshots or attempts to flee the violence. Authorities had earlier said that fewer than 10 people had non-life-threatening injuries