At least three Iraqi witnesses appeared before a federal grand jury Tuesday investigating last September’s killing of seventeen Iraqis by guards with the military company Blackwater Worldwide. No charges have been filed in what’s become known as the Nissor Square massacre. Blackwater is immune from prosecution in Iraq under a US-imposed law. Prosecutors say they’re still unsure whether Blackwater can be charged in the United States. One witness left the court building clutching a family photograph.