WASHINGTON (AFP) – In a rebuke to government prosecutors, a federal judge dismissed criminal charges against five Blackwater security guards accused of fatally shooting 14 people in Baghdad in September 2007.

Judge Ricardo Urbina said on Thursday prosecutors violated the defendants' rights by using incriminating statements they had made under immunity during a State Department probe to build their case.

"The government used the defendants' compelled statements to guide its charging decisions, to formulate its theory of the case, to develop investigatory leads, and ultimately to obtain the indictment in the case," Urbina ruled.

"In short, the government had utterly failed to prove that it made no impermissible use of the defendants' statements, or that such use was harmless beyond a reasonable doubt."

The security guards had been "compelled" to provide the incriminating evidence during a Justice Department probe, the court said, but the US Constitution bars the prosecutors from using "statements compelled under threat of a job loss" in any subsequent criminal prosecution.

The case was among the most sensational that sought to hold Blackwater employees accountable for what was seen as a culture of lawlessness and a lack of accountability as it carried out its duties in Iraq.

The five guards, who had been part of a convoy of armored vehicles, had been charged with killing 14 unarmed Iraqi civilians and wounding 18 others during an unprovoked attack at a busy Baghdad traffic circle using gunfire and grenades.

The men had faced firearms charges, and up to 10 years in jail on each of 14 manslaughter counts.

US prosecutors had alleged that the guards "specifically intended to kill or seriously injure Iraqi civilians," and according to court documents alleged that one of the guards told another that he wanted to kill Iraqis as "payback for 9/11," bragging about the number of Iraqis he had shot.

Urbina explained in his opinion that federal prosecutors were offered an opportunity during a three-week hearing that began in mid-October 2009 to prove that they had not made use of the defendants' statements in building its case and were unable to do so.

"The explanations offered by the prosecutors and investigators in an attempt to justify their actions... were all too often contradictory, unbelievable and lacking in credibility," Urbina wrote.

He added: "The court must dismiss the indictments against all of the defendants."

The five defendants were security guards employed by Blackwater Worldwide, which since has been renamed a Xe Corporation.

Attorneys for the guards have said they did not fire their weapons with criminal intent but thought they were under attack.

But critics repeatedly have accused the company of a Rambo-style "shoot first, ask questions later" approach when carrying out security duties in Iraq.

A State Department review panel in 2007 concluded that there had been insufficient US government oversight of private security firms hired in Iraq to protect diplomats and to guard facilities.

The panel found that as a result there was an "undermined confidence" in those contractors, both among Iraqis and US military commanders.