LINCOLN, Neb. (AP) — Officials at the Nebraska State Penitentiary say a series of inmate fights have occurred resulting in injuries to two staff members who were treated at a hospital for minor injuries and released.

Acting Warden Robert Madsen says in a statement he placed the prison on lockdown. Public activities including visits and volunteer events have been called off for the weekend.

He says the fights broke out at 7 a.m. Saturday and staff intervened to stop them. Some of the inmates needed medical attention.

Madsen says the prison is secure and an investigation has begun in to the incident.

The prison, located in Lincoln, is the state’s oldest correctional facility. It opened in 1869.