A U.S. federal judge has dismissed all charges against five Blackwater Worldwide security guards charged in a Baghdad shooting that killed 17 Iraqis.

District Judge Ricardo Urbina said the prosecutors ignored the advice of Justice Department officials and improperly built their case on sworn statements that had been given under a promise of immunity.

Blackwater contractors were hired to guard U.S. State Department diplomats in Iraq. Prosecutors say the guards fired on unarmed civilians at a busy intersection in 2007. Witnesses said the shooting was unprovoked, but Blackwater said its guards were ambushed by insurgents while responding to a car bombing.

After the shooting the guards gave statements to State Department investigators in which they admitted firing their weapons. That admission was crucial because forensic scientists could not match bullets from the shooting scene to specific weapons.

In exchange, the State Department promised the statements would not be used in a criminal case. The deal meant prosecutors had to build their case without them. Urbina said the Justice Department failed to do so.

Statements misused, judge finds

Prosecutors read those statements, reviewed them in the investigation and used them to question witnesses and get search warrants, Urbina found. Key witnesses also reviewed the statements and the grand jury heard evidence tainted by those statements, the judge said.

Urbina said the government's explanations for its actions were "contradictory, unbelievable and lacking in credibility."

The five guards had pleaded not guilty to manslaughter and weapons charges. A sixth guard turned on his former colleagues and pleaded guilty to killing one Iraqi and wounding another. It's unclear whether Urbina's ruling will allow him to get out of his plea deal.

"We're obviously disappointed by the decision," Justice Department spokesman Dean Boyd said. "We're still in the process of reviewing the opinion and considering our options." Prosecutors can appeal the 90-page ruling.

With files from The Associated Press