Gunmen opened fire on a prison in northern Brazil on Tuesday in an effort to aid a breakout, but an ensuing gun battle with police left 21 people dead, mostly inmates and their would-be liberators.
Of the 21 people who were killed at the Santa Izabel penitentiary in Belem, one was a guard; the other 20 were either prisoners or their supporters, authorities said.
Four other guards were injured in the prison-break attempt, officials said.
Describing the attempted mass prison break in Belem, a city located near the mouth of the Amazon River, the Secretariat of Public Security and Social Defense (SEGUP) of Pará said that the attack began with explosions at one of the prison's walls.
"The prisoners also had guns inside the prison," the agency said. A gun battle then broke out between, on one side, guards and police, and on the other, prisoners and the heavily armed group that was trying to engineer a mass escape.
The agency said the fighting at the prison had been intense and that the prison guards faced gunfire from inside and outside the prison walls.
NPR's Philip Reeves reports that some of the inmates did manage to escape, but officials did not say how many. They are thought to be heavily armed and police are on a manhunt for the escapees.
Philip notes, "Brazil's overcrowded prisons are dominated by drug gangs with access to guns and cell-phones and have a grim history of escapes, violence and massacres."
Last year, a prison uprising in Manaus in Brazil's Amazon region, left 56 dead during a 17-hour standoff. A total of 184 prisoners escaped, though many were eventually recaptured.
Brazil has among the world's largest prison populations, with nearly 650,000 inmates in 2016.