“Once you immunize statements, it is really hard to prosecute,” said Andrew Leipold, a law professor at the University of Illinois. “In the field, the people providing the immunity may value finding out what happened more than they do any possibility of prosecution. But that just makes any future prosecution really very hard.”

Justice Department officials declined to comment Wednesday about specific Blackwater cases. But the department has appealed the dismissal of the Nisour Square case, and a new trial has been scheduled for next March in the Virginia murder case after a mistrial was declared. And Justice officials noted that the government had had a number of successful prosecutions against contractors in Iraq and Afghanistan, including several for sexual assaults and other violent crimes. More than 120 companies have been charged by the Justice Department for contract fraud and related crimes in Iraq, Afghanistan and Kuwait, officials said.

Still, a Justice official who spoke on the condition of anonymity acknowledged that the government had faced tough obstacles. “There are substantial difficulties in prosecuting cases committed in war zones,” the official said. “There’s problems with the availability of witnesses, availability of evidence, and the quality of the evidence. You also have claims of self-defense, which are generally difficult, although not insurmountable.”

And self-defense is a more compelling argument in war zones, where many people are routinely armed.

One problem in the Moonen case, for example, was that while Mr. Moonen admitted in his statement to the embassy official that he did shoot the Iraqi guard, he asserted that he had done so in self-defense. The guards in the Virginia case also said that they shot in self-defense when they believed they were facing an attack from insurgents. In the Nisour Square case, the five Blackwater guards who were charged also claimed that they shot only after they believed they were under attack.

Jurisdictional problems also plague the Blackwater cases. Since the Blackwater guards were working under a contract with the State Department, they did not fall under the laws that govern contractors working for the Defense Department overseas. Contractors for the Defense Department are subject to criminal prosecution under the Military Extraterritorial Jurisdiction Act, but it has never been clear whether the law can be applied to contractors for the State Department, like Blackwater. Those contractors generally have greater protections because of the possibility that they might be engaged in fighting.

Until last year, foreign contractors also had immunity from prosecution under Iraqi law, so the Blackwater guards were operating in a legal vacuum, noted Eric Jensen, a law professor at Fordham University. “I would be concerned as a prosecutor that even if you got past the immunization, and the problems with witnesses and evidence, that you may not even have a law that supports the prosecution of a Department of State contractor,” Mr. Jensen said. “Congress has tried to address this, but it’s still a live question.”

Mr. Riley cited these reasons in a letter he wrote in April 2009 to Attorney General Eric H. Holder Jr. about the case and also noted that he believed the government had considered indicting Mr. Moonen to placate the Iraqi government. In a letter sent to Mr. Riley on Monday, notifying him that they were dropping the case, prosecutors also indicated that they would have difficulty proving their case beyond a reasonable doubt, particularly in overcoming Mr. Moonen’s claims that he shot in self-defense.

Meanwhile, the government said that the United States ambassador to Iraq, James F. Jeffrey, had to notify the Iraqi government of the decision, and also provided government officials a letter to be given to the family of the shooting victim, Raheem Saadoun. This year, Mr. Saadoun’s family dropped a civil lawsuit against Mr. Moonen and Blackwater after receiving a financial settlement.