LINCOLN, Neb. (AP) — The Nebraska State Penitentiary is still on lockdown after 33 inmates refused to return to their cells Monday night.

A Department of Correctional Services spokeswoman said Tuesday that the inmates were protesting a year-old policy that limits the number of inmates who can be in the prison yard at one time. The policy was enacted to reduce conflicts among inmates.

Spokeswoman Dawn-Renee Smith says there were no injuries or major damage in the protest, aside from some fires in trash cans that were extinguished. Some inmates were moved to other locations because of the smoke.

Smith says prison staff members are investigating the fires. The inmates who protested were not violent, and were all secured Monday night.