WASHINGTON

The gunman who killed four people at a hospital center in the US state of Oklahoma targeted and killed a doctor who recently performed surgery on him, police said Thursday.

Law enforcement discovered a letter on the suspected gunman, whom they identified as Michael Lewis, "which made it clear that he came in with the intent to kill Dr. (Preston) Phillips, and anyone who got in his way," Tulsa Police Chief Wendell Franklin told reporters. "He blamed Dr. Philips for the ongoing pain following the surgery."

Phillips performed back surgery on May 19 on the suspect and was released five days later, according to police. The suspect "called several times over several days complaining of pain, and wanted additional treatment" before Phillips saw him May 31.

Authorities said data from the Bureau of Alcohol, Tobacco, and Firearms indicates the suspect purchased an "AR-15-style" assault rifle hours before he carried out the mass shooting.

Law enforcement said Wednesday that the gunman used a handgun and a long rifle during the shooting spree before fatally shooting himself.

In addition to Phillips, the victims include Dr. Stephanie Husen, Amanda Green, and William Love, said authorities.

Police said in the aftermath of the shooting that they received a call about an active shooter at the Natalie Building at the St. Francis Hospital campus at 4:52 p.m. local time (2152GMT).

Officers arrived roughly four minutes later, hearing gunshots that took them to the crime scene on the building's second floor.