MEXICO CITY (AP) — A riot erupted inside a northern Mexico prison over prisoners trying to extort money from others and 56 inmates were inured, authorities said Saturday.

Officials said the riot began Friday at the Apodaca prison, just outside the city of Monterrey.

Aldo Fasci, the public safety secretary for Nuevo Leon state, said that the injuries were all caused by beatings and that no firearms were involved.

“There were no firearms, all the injuries were caused by blows, The head wounds were caused by blunt objects, either rocks or in some cases by (smashing them) on the floor,” Fasci said.

He said many inmates at the prison work in industrial plants and are usually paid on Friday. He said at least five gangs are extorting money from other inmates inside prison walls.

Authorities acknowledge that inmates have been in partial control of the facility.

“We have to keep working to eliminate shared rule” of prisons, Fasci said.

He said a sweep after the situation was brought under control found prohibited cellphones, beer and drugs among inmates’ possessions.