WASHINGTON (AP) -- Three Iraqis, including the father of a slain 9-year-old boy, appeared Tuesday before a federal grand jury investigating a deadly Sept. 16 shooting in Baghdad involving Blackwater Worldwide contractors.

The Iraqis were escorted to the closed-door session by federal prosecutors who are overseeing the U.S. investigation into whether Blackwater security guards illegally fired into a crowded Baghdad intersection, resulting in the deaths of 17 Iraqi civilians.

An Iraqi police major told The Associated Press in Baghdad that two of his officers were flown to the United States several days ago to testify. The major, who asked to remain anonymous because he was not authorized to speak publicly, said they were expected to remain in the United States for two weeks.

It was not known whether the officers, whose names were not provided by the police major, were among the three men meeting Tuesday with grand jurors at the federal courthouse in Washington.

One of the three Iraqis was Mohammed Abdul Razak, whose son Ali, 9, was killed in the shooting. He left court holding what appeared to be a family portrait. The men did not talk to reporters after about three hours behind closed doors but before leaving Iraq, Razak told ABC News he agreed to testify because he wanted justice for "a crime that needs to be punished."

"It was a true massacre, a slaughter," Razak said.

Grand jury testimony is secret but Iraqi witnesses to the shooting have described it publicly as an unprovoked attack in which the U.S. contractors killed motorists, bystanders and children.

Blackwater, hired by the State Department to guard U.S. diplomats in Iraq, says its contractors were responding to a Baghdad car bombing when they were ambushed by insurgents, touching off a firefight.

The company is not a target of the investigation. The case has focused on as few as three or four guards and whether they acted illegally.

Over the past seven months, the grand jury has heard from Blackwater security guards, company managers and U.S. military officials.

The shooting enraged the Iraqi government, which originally sought to expel Blackwater and its 1,000 employees from the country, and strained diplomatic relations between Washington and Baghdad.

The shooting also raised questions at home and abroad about the U.S. reliance on heavily armed private contractors in war zones.

(Copyright 2008 by The Associated Press.  All Rights Reserved.)