The guards were employed to protect US personnel in Iraq
Iraq has criticised a US judge's dismissal of all charges against guards from US security firm Blackwater over the killing of 17 Iraqis in 2007.
Government spokesman Ali al-Dabbagh said an Iraqi investigation showed the men had committed a "serious crime" and Baghdad would seek to prosecute them.
The five had all pleaded not guilty to manslaughter. A sixth guard admitted killing at least one Iraqi.
The judge dismissed the charges against the guards over procedural errors.
District Judge Ricardo Urbina said the US justice department had used evidence prosecutors were not supposed to have.
THE BLACKWATER INCIDENT
16 Sep 2007 - 17 Iraqi civilians killed in Nisoor Square, Baghdad
Iraq said - civilians fired on by Blackwater guards without provocation
Blackwater said - Iraqis killed in shootout when Blackwater convoy ambushed
Iraqi eyewitnesses - guards fired on white sedan that failed to slow down and it burst into flames, and then fired into surrounding area as people tried to flee
Nov 2007 - New York Times quotes FBI officials saying killings of at least 14 of the Iraqis were "unjustified"
Dec 2008 - five guards charged with 14 counts of manslaughter
Mr al-Dabbagh said the Iraqi government "regrets and is disappointed by the US court's decision".
"Inquiries carried out by the Iraqi government clearly confirm that the Blackwater guards committed a crime and used weapons when there was no threat necessitating the use of force," he said.
He said Iraq would "act forcefully and decisively to prosecute the Blackwater criminals".
The Iraqi human rights minister, Wejdan Mikhail, said she was "astonished" by the US move.
"There was so much work done to prosecute these people and to take this case into court and I don't understand why the judge took this decision," the AFP news agency quoted her as saying.
The commander of US forces in Iraq, Gen Ray Odierno, said the court's decision could create local resentment against other security firms operating in the country.
"Of course we're upset when we believe that people might have caused a crime and they are not held accountable," Reuters quoted him as saying.
The killings, which took place in Nisoor Square, Baghdad, strained Iraq's relationship with the US and raised questions about US contractors operating in war zones.
A man whose son died in the incident said he was surprised to hear the guards had been acquitted.
"But what can we do? We cannot do anything with the US government and their law," he told Reuters.
Lawyers for the five guards say they were acting in self-defence, but witnesses and family members of those killed maintain that the shooting on 16 September 2007 was unprovoked.
A civil case against Blackwater brought by Iraqis - including relatives of some of the Nisoor Square victims - is still before a Virginia court.
It alleges that Blackwater employees engaged in indiscriminate killings.