At least 21 people died Tuesday in an attempted breakout from a prison in northern Brazil aided by an armed group on the outside trying to blow up a wall, officials said.
The Para state security service described the mass escape attempt at the Santa Izabel Penitentiary Complex, near the Amazon rainforest city of Belem in Para state, as a military style battle in which both prisoners inside and associates on the outside were armed.
"In the rescue attempt, explosives were used against one of the walls," the agency said in a statement.
"The attempt was supported by an extremely heavily armed group. According to preliminary reports, the prisoners also had weapons inside the prison," the statement said.
Prison guards fought back in "an intense exchange of gunfire," it said.
The confirmed dead so far included 20 prisoners and outside attackers, plus one guard. Another five guards were injured, including one seriously.
A headcount was underway to establish whether anyone escaped, while a police SWAT team was reinforcing security.
Police had recovered two rifles and five handguns at the scene, the statement said.
Brazil is one of the world's deadliest countries, with around 60,000 homicides a year, and the prisons are notoriously overcrowded and violent.