“We had high hopes with the previous case,” Mr. Sadoon said. “The case got to a point where we thought it could succeed, but then everything failed and we gave up on the case.”

As Mr. Sadoon spoke, cars bustled through the square, which is surrounded by a traffic circle that connects many of Baghdad’s main streets. Police officers blew whistles to direct drivers, who honked their horns loudly as they tried to make their way through the city’s unrelenting traffic.

“They can open the case, but I think in the end there will be political pressure to close it again,” Mr. Sadoon said. “We have learned our lesson from what happened before with the case.”

The Nisour Square shootings enraged Iraqis, setting off anti-American sentiment that eventually led the Iraqi government to subject all foreign contractors to the country’s laws.

In 2008, the United States Justice Department indicted the guards on manslaughter charges based on sworn statements the guards had given to the State Department under the promise of immunity.

One guard, who pleaded guilty and agreed to testify against the others, said the guards had opened fire with automatic rifles and grenade launchers and that there had been no threat to the convoy.

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But in 2009, a federal judge in Washington threw the case out, saying that the Justice Department’s use of the statements the guards gave in exchange for immunity compromised their right to a fair trial.

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The dismissal once again enraged Iraqis, who believed that the United States had allowed the contractors to operate above the law. In a visit to Iraq, Vice President Joseph R. Biden Jr. responded by saying that “a dismissal is not an acquittal,” and the Obama administration appealed the decision.

Not surprisingly, all of the legal back and forth has left many victims confused.

When news that the case had been reopened broke on Friday, there were no bulletins on television. Many victims first learned of the ruling from reporters, who called them for comment.

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Local newspapers, meanwhile, had no articles on the ruling until Sunday, and did not give them much prominence.

Iraqi politicians, including Prime Minister Nuri Kamal al-Maliki, had harshly criticized the dismissal of the charges in 2009 but in this instance said little.

“There are no reactions so far because the issue is still confusing to many Iraqis,” said Ali al-Moussawi, a top adviser to Mr. Maliki. “We are waiting to have something clear in this regard.”

However, Mr. Moussawi said the government had confidence in the American judicial system. “We welcome this decision and consider it evidence of the fairness in U.S. judiciary,” he said.

He added, “It’s the Iraqi government point of view that opening the case again will give the victims who died some justice.”

Sami Hawas, 46, was driving with his mother and son in 2007 when they were stopped by the police at Nisour Square, which was under construction after it had been bombed earlier that year.

The police, Mr. Hawas said, had stopped him because an American convoy was passing through.

Several Blackwater sport utility vehicles traveling on the wrong side of the road came toward his car and began firing, Mr. Hawas said.

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“I got shot all over my body,” he said, including several wounds to his back and right leg. “I didn’t know where the bullets were coming from.”

Mr. Hawas, a taxi driver, said he could no longer drive without pain from his injuries. “We don’t understand what is going on,” he said, referring to the case.

He added: “I guess nothing will be changed because this is not the first case in which innocent people have been killed. And people’s lives are priceless. No one can compensate for those losses.”