Police in Northern Ireland covered up murders and other crimes committed by Protestant loyalist gangs in the 1990s, an investigation revealed this week. The police ombudsman, Nuala OLoan, said police protected their informants in the paramilitary group Ulster Volunteer Force, even when they knew those informants had murdered people. OLoan said the dirty cops could not now be charged, because most of the documents incriminating them had been destroyed. The Ulster Volunteers were the Protestant answer to the Irish Republican Army, the Catholic militant group that sought to sever Northern Irelands union with Britain. The police report is expected to complicate efforts to form a power-sharing government in Northern Ireland, where many Catholics do not trust the authorities.