Fifty of the 105 inmates who escaped from a maximum-security prison in Brazil's Paraiba state have been apprehended while the 55 others still remain at large, officials said.On Monday, some 20 men armed with rifles and explosives arrived at the prison in four vehicles and opened fire on guard posts, the barracks and the main entrance of the Romeu Goncalves Abrantes maximum-security prison in Joao Pessoa, the state capital, reports Efe news.Guards and police engaged the gunmen in a shootout, authorities said, adding that the prison's main door was then knocked down, allowing 105 prisoners to escape.Another group of gunmen, meanwhile, blocked part of a nearby highway and shot a 36-year-old police officer in the head.The officer was transported to a hospital, where he remains listed in critical condition.The Federal University of Paraiba, some schools and hospitals suspended operations as a precaution to "ensure the safety of clients and professionals," the Joao Pessoa Mayor's Office said.Security was heightened in the city as officers sought to track down the prisoners still at large.