LINCOLN, Neb. (AP) — Officials say roughly a dozen inmates were involved in an attack on nine staff members at a Nebraska prison.

Lincoln Correctional Center spokeswoman Tammy Kluver said Thursday that investigators are trying to determine exactly what happened. The Nebraska State Patrol is reviewing video footage of the incident that took place Wednesday evening in a small courtyard.

Prison officials say the inmates involved had refused to return to their cells.

Kluver says the injured employees were treated at hospitals and released. One female staff member required staples to close a gash in her head. Authorities used pepper spray to subdue the inmates.

Gov. Pete Ricketts plans to meet with prison officials Thursday to discuss the incident.

The Lincoln Correctional Center, a medium-maximum security prison for adult males, remains on lockdown.