In a 90-page opinion, U.S. District Judge Ricardo M. Urbina ruled that the government violated the guards' rights by using their immunized statements to help the investigation. The ruling comes after a lengthy set of hearings that examined whether federal prosecutors and agents improperly used such statements that the guards gave to State Department investigators following the shooting on Sept. 16, 2007.

"The explanations offered by prosecutors and investigators in an attempt to justify their actions and persuade the court that they did not use the defendants' compelled testimony were all too often contradictory, unbelievable and lacking in credibility," Urbina wrote.

Dean Boyd, a spokesman for the Justice Department, said, "We're obviously disappointed by the decision. We're still in the process of reviewing the opinion and considering our options."

The five guards -- Paul Slough, Nicholas Slatten, Evan Liberty, Dustin Heard and Donald Ball -- are charged with voluntary manslaughter and weapons violations in the killing of 14 civilians and the wounding of 20 others.

The Justice Department alleges that the guards unleashed an unprovoked attack on Iraqi civilians in Nisoor Square while in a convoy. One guard, Jeremy P. Ridgeway, has pleaded guilty and was expected to testify against the others. Blackwater, which has since renamed itself Xe, had a contract to provide security for the State Department in Iraq.

Mark Hulkhower, the defense lawyer representing Slough, said he was obviously pleased. "We are very gratified by the judge's thoughtful and reasoned opinion and we are very happy that Mr. Slough can start the New Year without this cloud hanging over his head."