Three Iraqis, including the father of a dead nine-year-old boy, appeared before a US grand jury investigating the deadly shooting in Baghdad involving Blackwater Worldwide contractors.
The Iraqis were escorted to the closed-door session in Washington by prosecutors overseeing the US investigation into whether Blackwater security guards illegally fired into a crowded Baghdad junction on September 16 last year, resulting in the deaths of 17 Iraqi civilians.
An Iraqi police major said in Baghdad that two of his officers were flown to the US several days ago to give evidence. The major said they were expected to remain in the US for two weeks.
It was not known whether the officers, one of whom was identified as Serhan Dhiab, were among the three men meeting grand jurors at the Washington federal court.
One of the three Iraqis was Mohammed Abdul-Razzaq, whose son Ali, nine, was killed in the shooting. He left court holding what appeared to be a child's toy and a family portrait.
After about three hours behind closed doors, the men did not talk to reporters. But before leaving Iraq, Mr Abdul-Razzaq said he agreed to testify because he wanted justice for "a crime that needs to be punished".
"It was a true massacre, a slaughter," Abdul-Razzaq told ABC News.
ABC identified Hussan Abdurrahman as one of the officers brought to testify. He told the network in a separate interview that the Blackwater convoy never was in danger.
"There were zero armed men in that area," Mr Abdurrahman said.
Grand jury testimony is secret, but Iraqi witnesses to the shooting have described it publicly as an unprovoked attack in which the US contractors killed motorists, bystanders and children.
Blackwater, hired by the State Department to guard US diplomats in Iraq, says its contractors were responding to a Baghdad car bombing when they were ambushed by militants, starting 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 US 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 US reliance on heavily armed private contractors in war zones.