Boa Vista - Brazil was hit on Friday by its second grisly prison massacre in a week, as inmates beheaded and mutilated fellow prisoners at a northern jail, leaving 31 dead.Pictures taken by a police officer at the scene showed bloodied, mangled bodies piled in a concrete hallway at the Monte Cristo Farm Penitentiary (PAMC) in Roraima state.Many of the victims were beheaded, disembowelled or dismembered, officials said.The state government said the situation was now "under control".They lowered an earlier death toll of 33 to 31 late on Friday."No shots were fired. The victims were killed with sharp objects and home-made weapons," said state justice secretary Uziel Castro.Gang infighting?The latest unrest came days after jailed gang members killed 56 rivals in a 17-hour bloodbath on Sunday and Monday at a prison in Manaus, the capital of the neighbouring state of Amazonas.Unlike that incident, Friday's violence did not appear to be an all-out riot but rather a rapid, early morning attack by one group of inmates against another, lasting less than an hour, a local government spokesperson told AFP.Most of the killings were carried out with knives, she said.Deadly prison riots have intensified in Brazil since a truce broke down in July between the country's two largest drug gangs, the First Capital Command (PCC) and the Red Command (CV).PAMC, the largest prison in the state, was also hit by deadly violence in October, when fighting between rival gangs killed 10 inmates.At the time, the prison held 1 400 inmates - double its capacity.'Dreadful conditions' Authorities said the Manaus riot was caused by a local ally of the Rio de Janeiro-based CV massacring rivals from the Sao Paulo-based PCC.They said however that Friday's killings appeared not to have been retaliatory, said Justice Minister Alexandre de Moraes."It is apparently not a revenge attack by the PCC" for Monday's massacre, after which members of rivals gangs were moved to separate blocks.Speaking in Brasilia before travelling to the scene, he said PCC members were thought to have attacked prisoners who were not members of any gang.Castro said it was thought to be "an atrocity committed against regular prisoners.""The dreadful conditions in the prison are no secret to anyone," he added.