Prison officials in Nebraska say nine staff members at the Lincoln Correctional Center have been assaulted by inmates who were refusing to return to their cells.

Department of Correctional Services spokeswoman Dawn-Renee Smith told The Associated Press in an email Wednesday night that all nine were taken to hospitals after the attack and have been treated and released.

The prison has been placed on lockdown. It's not clear how many inmates were involved in the Wednesday evening attack or whether any were hurt.

The Lincoln Correctional Center is a medium-maximum security prison for adult males.