GRAND RAPIDS, Mich. (AP) -- Authorities say jail inmates in Grand Rapids rescued an officer who was being attacked by another inmate.

Kent County Undersheriff Jon Hess says 20-year-old Willie Lee Williams attacked the female jail officer early Friday. He says Williams didn't have a weapon, but he describes the incident as a serious assault.

The officer was treated and released from a hospital. Hess declined to release her name or any details about her injuries.

Hess says Williams was hiding in a room typically used by inmates during the day. The undersheriff says other inmates were able to help the officer because Williams struck an electrical panel that opened cells.

Williams is in jail facing robbery charges. He'll now face additional charges.