Justice Department officials have told Congress that they face serious legal difficulties in pursuing criminal prosecutions of Blackwater security guards involved in a September shooting that left at least 17 Iraqis dead. In a private briefing in mid-December, officials from the Justice and State Departments…warned…that there were major legal obstacles that might prevent any prosecution…[O]ne [problem] arose when State Department investigators granted Blackwater employees a limited form of immunity for what they disclosed. There are also questions about whether federal law applies to the Blackwater contractors. Justice officials have acknowledged the potential problems, but they have said they were not insurmountable and reiterated that point on Tuesday.