Tulsa Police Chief Wendell Franklin identified Michael Louis (some media outlets report as Michael Lewis), 45, of Muskogee, Oklahoma, as the gunman who shot and killed four people on the second floor of the Natalie Medical Building at Saint Francis Health System on Wednesday evening, before reportedly taking his own life.

Franklin said around 4:52 pm local time, Louis opened fire on the second floor of the medical building, targeting a doctor (Dr. Preston Phillips) who operated on his back on May 19.

Franklin said Louis complained to the hospital multiple times about pain after the surgery, though it's unclear what Lewis requested. 

Officers recovered a letter written by the suspect with his clear intent to kill Dr. Phillips and "anyone who got in his way."

Franklin shared the names of the four victims Louis killed, including Dr. Phillips, Dr. Stephanie Husen, Amanda Glenn (receptionist), and William Love (a patient). 

Louis was armed with an AR-15-style rifle he bought the same day and a .40-caliber Smith & Wesson semi-automatic handgun on Sunday. 

"The information that we have at the current time is that they were legally purchased firearms, one purchased an hour and some change before the actual shooting event took place, and the other purchased three days before the shooting took place," Franklin said.