Twenty people, including a guard, died when the inmates of a prison in the North Brazilian city of Belem attempted to break out on Tuesday, BBC reported. Four guards were injured, and one them is reportedly in a serious condition.

An armed gang reportedly aided the prisoners from outside the jail by setting up explosives against one of the walls. It was not clear how many of the dead were prisoners and how many were members of the gang that tried to free them.

Police officer Rodrigo Leao said two assault weapons, three pistols and two other guns were seized from the gang that attempted to help the prisoners, AP reported.

Brazil has one of the largest inmate populations – more than 7,26,000 prisoners – in the world, and often has to deal with large-scale violence in jails. Nine inmates of a jail in the central Brazilian state of Goias died in January during a clash between rival gangs. At least 56 inmates at a prison complex in Brazil’s Amazonas state were killed during a riot between rival drug gangs in January 2017.