STILLWATER— 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. Minnesota Public Radio News says no other details were released.

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.