A Minnesota prison was placed on emergency lockdown on Sunday after about 100 prisoners refused to return to their cells, authorities say.

The Minnesota Correctional Facility – Stillwater houses roughly 1,200 inmates, and around 100 inmates participated in the group protest. A spokesperson told the Associated Press that the reason for the prisoners’ refusals “remains unclear.”

The Minnesota Department of Corrections told FOX 9 Minneapolis that the prison’s staff members were removed from common areas. A Special Operations Response Team was also deployed to handle the crisis.