Nearly four years after the federal government began a string of investigations and criminal prosecutions against Blackwater Worldwide personnel accused of murder and other violent crimes in Iraq and Afghanistan, the cases are beginning to fall apart, burdened by a legal obstacle of the government’s own making…Interviews with lawyers involved in the cases, outside legal experts and a review of some records show that federal prosecutors have failed to overcome a series of legal hurdles, including the difficulties of obtaining evidence in war zones, of gaining proper jurisdiction for prosecutions in American civilian courts, and of overcoming immunity deals given to defendants by American officials on the scene…The difficulty of these cases also illustrates the tricky legal questions raised by the government’s increasing use of private contractors in war zones.