Tulsa police said on Thursday that a gunman, who killed four people at a medical building in the Oklahoma city, went to the facility to kill a doctor he blamed for the pain he experienced after surgery.

The gunman, identified as Michael Lewis, entered a building on the St. Francis Health System campus with an AR-15-style weapon and opened fire, killing two doctors and two other people, Tulsa Police Chief Wendell Franklin said at a news briefing.

Lewis, who killed himself, left a letter at the scene, Franklin said.

In addition to the four fatalities, several others were injured.

The shooting comes on the heels of two mass shootings that have stunned Americans and reopened a long-standing debate over tightening controls on firearms ownership and the role of mental health in the epidemic of gun violence plaguing the country.