LINCOLN, Neb. (AP) — A Nebraska prison has been placed on lockdown and inmates in a housing unit are refusing to return to their cells.

Officials say approximately 40 of unit’s 128 inmates are involved in the disturbance at the Tecumseh State Correctional Institution. A prison spokeswoman says a fire is burning in a prison yard, but authorities do not see any signs of fire inside the unit.

No injuries have been reported. Department spokeswoman Dawn-Renee Smith says all staff members are safe. Smith says the incident is isolated to half of the housing unit and a small fenced yard, and poses no risk to the public.

The prison in southeast Nebraska was the site of a May 2015 riot that injured several staff members, caused widespread damage and left two inmates dead.