A federal judge in Washington arraigned all five guards on manslaughter charges Tuesday.
They are charged with 14 counts of manslaughter, 20 counts of attempted manslaughter and one count of using a machine gun to commit a crime of violence. The machine gun charge, typically used in drug cases, carries a 30-year minimum prison sentence.
It stems from a shooting last year. The shooting took place around noon on Sept. 16, 2007, in a crowded square where prosecutors said civilians were running errands, getting lunch and otherwise going about their lives.
Prosecutors say the guards opened fire on unarmed civilians including women and children.
Twenty others were wounded in the crowded square, including one injured by a grenade launched into a nearby girls' school. Another 18 Iraqis were assaulted but not wounded, prosecutors said.
The guards claim they were attacked first.
North Carolina-based Blackwater is the largest contractor providing security in Iraq. Most of its work for the State Department is in protecting U.S. diplomats in Iraq. The company has not been charged in connection with the shooting.