(CNN) -- Four former Blackwater private security contractors faced new federal charges on Thursday, the latest chapter in a controversial political and diplomatic case over a deadly 2007 Iraqi War incident.

The men were re-indicted by a federal grand jury in Washington on various manslaughter charges related to a shooting that left 17 unarmed Iraqi civilians dead and at least 18 others wounded at Nisur Square in Baghdad.

The indictment alleges the men "unlawfully and intentionally, upon a sudden quarrel and heat of passion, did commit voluntary manslaughter."

The defendants have denied all charges and said they acted in self-defense when fired upon initially by militants.

A federal judge in 2008 dismissed the original case, finding the Justice Department withheld key evidence, and violated the guards' rights. But a federal appeals court later reversed that decision, leading the government to seek a fresh indictment.

The incident prompted international outrage at the time, and the criminal prosecution in the United States has dragged on for years.

The killings led Iraq's government to slap limits on security contractors hired by various firms, including Virginia-based Blackwate r-- now under new management and known as Academi.

"The vast majority of the U.S. contractors who served in Iraq did so with honor and integrity, but, as alleged today, these defendants abused their power through a relentless attack on unarmed civilians that recklessly exceeded any possible justification," said U.S. Attorney Ronald C. Machen Jr.

"This prosecution demonstrates our commitment to upholding the rule of law even in times of war and to bringing justice to the memories of those innocent men, women, and children who were gunned down in Baghdad more than six years ago," he said.

There was no immediate reaction from attorneys for the men. There was no indication when a new criminal trial would begin.

The case is U.S. v. Slough, et al (cr-08-360).