Authorities in Brazil’s Rio de Janeiro state have issued arrest warrants for nine people in connection with an extortion ring that demanded bribes from those visiting inmates.

The extortion ring operated at Polinter prison in Nova Friburgo, northeast of Rio’s state capital, and involved prison officers and inmates. Four have been arrested in “Operation Pharaoh,” including a prison administrator, and arrest warrants have been issued for 12 others.

While carrying out a property search at the home of one of the suspects, police discovered a large supply of suspected contraband. All suspects will be charged with criminal conspiracy to commit bribery. The prison administrator and police guards will also be charged with extortion by a public official.

Brazil’s prisons are notoriously overcrowded and difficult to manage, and extortion rings operated by prisoners have been identified in the past.