STILLWATER, Minn. (AP) — The Minnesota Department of Corrections is investigating recent attacks on 11 prison officers.

The latest happened Thursday when an offender assaulted a lieutenant at the Minnesota Correctional Facility-Stillwater. The officer walked out of the facility on his own and was treated for non-life-threating injuries. He was not admitted to a hospital. .

Last weekend, a total of 10 officers were assaulted in two separate incidents at the Oak Park Heights maximum security prison. Five workers were hurt during a struggle with a prisoner Sunday night. That happened two days after five employees were injured during an inmate melee at the prison Friday night.

Department of Corrections Commissioner Tom Roy says offender violence inside a prison "is simply not acceptable."