Two prisoners were beheaded and another killed during a riot over conditions at a jail in southern Brazil.
Inmates of the penitentiary in Cascavel took at least two other agents and a number of other inmates hostage in the uprising on Sunday, according to military police captain Ricardo Pinto. He said negotiations for better conditions at the 928-bed prison were still under way 12 hours later.
Prisoners are reported to have set fire to objects and used metal poles to wreak damage at the jail, which housed more than 1,000 inmates.
Authorities initially said two men were decapitated, and later learned of a third prisoner who was also killed after he was thrown off the roof of a building. The three men were not identified.
The revolt began before sunrise when a prison guard was captured during breakfast, Pinto said.
Video: Prisoners take over prison and kill 4 inmates
Dozens of the prisoners climbed onto the building's rooftop, with their faces covered with white fabric. Local media images showed at least 30 rebellious inmates shouting while they beat men held with ropes around their necks, or whose hands were tied behind them.
The rioting inmates waved banners emblazoned with the initials PCC for a criminal prison gang formed in the 1990s.
Jairo Ferreira, a lawyer for the prison guards' union, told local news site CGN that at one point the inmates put the decapitated head of one victim on the lap of a custodian who was initially held hostage and later freed.
Relatives arriving at the prison to visit inmates waited outside as night fell, trying to get information about their loved ones.
Ferreira said the prisoners rioted to demand better food and medical care in the prison.
Pinto said the revolt may also have been related to the recent convictions of two prisoners.
Last year, a prison rebellion in São Luis, in the north-eastern state of Maranhão, Brazil, left 13 dead and more than 30 injured.
And last month, Human Rights Watch said in a letter to the Brazilian Congress that it had found compelling evidence in 64 cases of alleged abuse since 2010 that security forces or prison authorities "engaged in cruel, inhuman, or degrading treatment" against people in their custody.
Additional reporting by agencies