WASHINGTON - The Justice Department has obtained indictments against five guards for the security company Blackwater Worldwide for their involvement in a 2007 shooting in Baghdad that killed at least 17 Iraqi civilians and remains a thorn in Iraqi relations with the United States.

The indictments, obtained Thursday, remained sealed. But they could be made public in Washington as soon as Monday, according to people who have been briefed on the case and who spoke on condition of anonymity because the indictments had not been unsealed.

A sixth guard was negotiating a plea, those people said.

Peter A. Carr, a spokesman for the Justice Department, declined to comment on Friday. Anne E. Tyrrell, a spokeswoman for Blackwater, also declined to comment.

The six guards have been under investigation since the shootings occurred Sept. 16, 2007, as their convoy traveled through a traffic circle in Nisour Square that was filled with cars, pedestrians and police officers. The guards have told investigators that they fired after coming under attack. Blackwater has maintained that its guards did nothing wrong, and the company itself is not being charged in the case. Investigations by the Pentagon, the F.B.I. and the Iraqi government found no evidence to support the guards' version of events.

Among those named in the indictment, according to the people briefed on the case, are Paul Slough, a 28-year-old who served in the Army Infantry and the Texas National Guard before joining Blackwater in 2006, and Dustin Heard of Tennessee, a former marine who joined Blackwater in 2004.

Those who have been briefed on the case said prosecutors could seek 30-year prison sentences under a Reagan-era antidrug law focusing on the use of machine guns in the commission of violent crimes. Drugs were not involved in the Blackwater case.

Mark Hulkower, Mr. Slough's lawyer, would not confirm whether his client was one of those indicted. But if he is, Mr. Hulkower said, "We will contest the charges in court, and we are confident he will be vindicated."

The Nisour Square shootings have had a profound impact in Iraq, both on the role of contractors in the war zone and on the Baghdad government's relationship with the Bush administration. The episode was the bloodiest in a series of violent events involving Blackwater and other American security contractors that had stoked anger and resentment among Iraqis.

Founded in 1997 by Erik Prince, a former member of the Navy Seals and heir to a family fortune made in the auto parts industry, Blackwater had developed a reputation among Iraqis and American military personnel for flaunting an aggressive, quick-draw image and for security personnel who took excessively violent actions to protect the people they were paid to guard.

In December 2006, a Blackwater guard who was off duty and reportedly drinking heavily was reported to have shot a bodyguard for an Iraqi vice president in Baghdad. In 2007, the State Department acknowledged that Blackwater had been involved in many more shootings than the two other security contractors in other regions of Iraq.

But the Nisour Square episode prompted so much protest that Iraq's prime minister, Nuri Kamal al-Maliki, demanded that the Bush administration pull Blackwater out of the country.

In a profile of Mr. Slough, The New York Times reported this year that he had used dry military language to explain to investigators that he fired his weapon only at targets who posed immediate threats to his life and to those of his colleagues.

He described fighting his way out of a terrifying ambush that began when the driver of a white, four-door sedan ignored numerous hand signals and drove directly at the Blackwater motorcade. And he described muzzle flashes from a shack about 160 feet behind the car, a man in a blue button-down shirt and black pants pointing an AK-47, small-arms fire from a red bus stopped in an intersection, and a red car backing up toward his convoy.

"I engaged the individuals," Mr. Slough told investigators, "and stopped the threat."

The F.B.I. concluded that at least 14 of the 17 fatal shootings in Nisour Square were unjustified, saying that Blackwater guards recklessly violated American rules for the use of lethal force. Military investigators went further, saying that all of the deaths were unjustified and potentially criminal. Iraqi authorities characterized the incident as "deliberate murder."

Still, the guards could not be prosecuted under Iraqi law because of an immunity agreement signed by the Coalition Provision Authority, the governing authority installed by American troops after the invasion. And legal experts have long pointed out that the case faces significant legal hurdles in American courts, which have only vague powers to prosecute Americans for crimes committed abroad.

Immunity for security contractors became a central issue this year in the negotiations between Iraq and the United States over an agreement setting out the terms under which American troops could remain in Iraq. Iraqi officials repeatedly demanded an end to legal immunity for American contractors. The Bush administration eventually agreed, and tens of thousands of contractors will be held responsible for their actions under Iraqi law at the start of next year.