STILLWATER, Minn. (AP) -

Stillwater state prison is on lockdown after officials say an inmate attacked and injured four staff members without warning.

A spokesman for the Minnesota Department of Corrections says the altercation began Wednesday when the offender approached staff members with a complaint, then attacked them.

Additional officers were sent to the scene and the offender was taken into custody.

The incident happened in the prison's dining hall about midday. The corrections spokesman tells the Star Tribune a female correctional officer and a contract food service employee were treated at a hospital and released. Two other employees had minor injuries.

A criminal investigation of the incident is underway.