A US judge dismissed all charges against the security guards last month, saying that US justice department prosecutors improperly built their case on sworn statements that had been given under a promise of immunity.
Iraqi officials had denounced the court ruling and said that they would file lawsuits in US and Iraqi courts against the security firm.
The guards, who had been part of a convoy of armoured vehicles, had been charged with killing 14 unarmed Iraqi civilians and wounding 18 others in September 2007 at a busy Baghdad roundabout using guns and grenades.
But the guards claimed they acted in self-defence after a convoy they were protecting near Nisour Square came under attack.
However, witnesses and victims say the guards shot indiscriminately.
The five guards, Donald Ball, Dustin Heard, Evan Liberty, Nick Slatten and Paul Slough, all formerly in the US military, had been charged with manslaughter and weapons charges, which carried mandatory 30-year prison terms.
The incident through a spotlight on US security firm Blackwater, which had been hired to guard US diplomats in Iraq at the time and has since changed its name to Xe Services.
The firm pulled out of Iraq in May last year, after the US state department refused to renew its contracts.