An inmate at the Stillwater prison assaulted a correctional officer on Saturday morning, according to the Minnesota Department of Corrections. The department said a second officer intervened and was then assaulted by a second inmate.

No weapons were used and the officers were treated for minor injuries, department spokesperson Shannon Loehrke said in an email.

The prison is in Bayport, just southeast of Stillwater, about 25 miles (40 kilometers) east of Minneapolis. It houses about 1,200 inmates, according to department records.

The department said that the two inmates involved were transported to the state’s maximum security prison, and the Stillwater prison is on lockdown while an investigation is underway.