Five security guards have been charged with killing 14 unarmed civilians and wounding 20 others in a 2007 shooting in Baghdad.
The five, all former US servicemen, were security guards, or so-called 'contractors', for the US firm Blackwater Worldwide in Iraq at the time of the shooting.
They were charged with manslaughter, attempt to commit manslaughter and weapons violations.
A sixth Blackwater guard pleaded guilty on Friday.
The shooting by the private security firm's guards as they escorted a convoy of US diplomats through Baghdad on 16 September 2007, ignited a deep outcry in Iraq and led US officials to tighten controls on guards and contractors to tone down their tactics.
It is not clear whether under US law the contractors can be The law is murky on whether the contractors can be tried in US courts for crimes committed overseas.
Under the security pact recently agreed between Iraq and the US, foreign contractors, including some 30,000 mercenaries, will become subject to trial in Iraqi courts with effect from 1 January 2009.