They are accused of firing on 17 Iraqi civilians in Nisoor Square, in September 2007.

Their identites are Evan Liberty and Donald Ball, both 26-year-old former marines,  Dustin Heard, a 27-year-old ex-marine, Nick Slatten, 25, an ex-army sergeant, and Paul Slough, a 29-year-old army veteran.

Disputed case

Blackwater said that the guards were returning fire after their convoy was shot at. The head of Blackwater even appeared before the US Congress shortly after the attack, saying that the men acted responsibly.

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Blackwater on trial

Agents from the FBI found in investigations in late 2007 that most of the 17 deaths had been unjustified.

The incident created a furor about the perceived ability of private guards to act with impunity in Iraq.

An Iraqi government spokesman has said that they believed that the attack were tantamount to deliberate murder.

The case is complicated by the sphere of jurisdiction over private security guards not being clear.

At the time of the attack, private contractors like Blackwater operated without any clear legal oversight and it could be argued they did not have to answer either to Iraqi or US laws.

Poor forensic evidence

Under the deal Blackwater had with the US government, it was allowed to repair the vehicles involved in the attack before investigators saw them, taking away key forensic evidence.

But a sixth man, also a suspect in the case who has not been identified is expected to plead guilty to lesser charges in exchange for his cooperation in testifying against his former colleagues, which may help the prosecution.

Riad Kahwaji, the Middle East bureau chief for the international defence publication, Defence News, told Al Jazeera: "So far, in light of the investigations, it seems as if the right decision has been made, and these people have to answer to a court of law."

Kahwaji said that it is not certain that the men will make trial - first a federal judge has to evaluate the evidence before approving the case being taken to court.

He said that the move would be good for US-Iraq relations even if there are complications during any trial: "The US would have shown that it is willing and ready to take action when need be against civilian contractors."