The Justice Department brought new charges Thursday against four former Blackwater Worldwide security guards accused of taking part in a shooting in Baghdad six years ago that killed 14 unarmed civilians, wounded 18 and enraged public opinion in Iraq.

A federal grand jury in the U.S. District Court for the District of Columbia returned a fresh indictment charging the four guards with voluntary manslaughter and other crimes in the shooting in Nisour Square.

The guards were providing security under a State Department contract for diplomats in Iraq at the time of the shooting. On Sept. 16, 2007, they were part of a four-vehicle convoy that was securing an evacuation route for U.S. officials fleeing a bombing. The guards told U.S. investigators that they opened fire on the crowd in self-defense.

In a long investigation after the attack, the FBI and federal prosecutors concluded that the shooting was an “unprovoked illegal attack” on civilians.

“Today’s indictment charges four Blackwater guards with killing or wounding 32 defenseless Iraqi citizens, including women and children, in a Baghdad traffic circle in September 2007,” U.S. Attorney Ronald C. Machen Jr. said in a statement. “These defendants abused their power through a relentless attack on unarmed civilians that recklessly exceeded any possible justification.”

Charges were first brought against six Blackwater guards in 2008.

In 2009, U.S. District Judge Ricardo M. Urbina said that federal investigators and prosecutors had obtained the indictment by using tainted evidence. He concluded that government lawyers improperly used statements that the guards gave the State Department in the hours and days after the shooting with the belief that they wouldn’t be used in court.

Urbina threw out all the initial charges and issued a scathing opinion, saying that the federal prosecutors’ conduct was so egregious that it “requires dismissal of the indictment against all the defendants.”

Two years ago, the U.S. Circuit Court of Appeals for the D.C. Circuit revived the prosecution by ruling unanimously that Urbina had misinterpreted the law. Appellate Judges Douglas H. Ginsburg, Merrick B. Garland and Stephen F. Williams found that Urbina had “made a number of systemic errors based on an erroneous legal analysis.”

One of the contractors, Jeremy Ridgeway, pleaded guilty to manslaughter charges in 2008 and cooperated with the government. Last month, prosecutors dropped charges against another guard, Donald Ball.

The new charges were brought Thursday against the four other guards, all military veterans: Paul A. Slough, 34; Nicholas A. Slatten, 29; Evan S. Liberty, 31; and Dustin L. Heard, 32. They pleaded not guilty when previously charged.

“We are disappointed that the Department of Justice has chosen to proceed with this prosecution, which we strongly believe has no merit whatsoever,” said attorney David Schertler, who represents Heard. “We will continue to fight and defend Dustin Heard’s innocence and honor until he is fully exonerated.”

The Nisour Square shooting damaged the reputation of Blackwater, which changed its name to Xe Services and then Academi. Erik Prince, who founded Blackwater, is no longer involved with the company.

Machen said Thursday that the vast majority of the U.S. contractors who served in Iraq did so with “honor and integrity.”

“A limited number of members of the Blackwater team unleashed powerful sniper fire, machine guns, and grenade launchers on ordinary people going about their daily lives,” he said in his statement. “This prosecution demonstrates our commitment to upholding the rule of law even in times of war.”

Julie Tate contributed to this report.