A US government prosecutor on Tuesday chronicled for a jury the alleged conduct of four Blackwater security guards accused of killing 14 Iraqis and wounding 18 others in downtown Baghdad nearly seven years ago.

In opening statements at the trial of the four guards, assistant US attorney T Patrick Martin said some of the victims were "simply trying to get out" of the way of gunfire from Blackwater guards. "Fourteen died, 18 injured. For what?" he said.

One component of the prosecutors' case is that the Blackwater guards harbored deep hostility toward Iraqis and boasted of indiscriminate firing of their weapons.

Immediately after the shootings at Nisoor Square on 16 September 2007 as soon as the guards got back to their base, they participated in a lie that there were insurgents in the area, said Martin

"That lie that they had begun that day would unravel within moments" because two veteran army officers showed up on the scene to see what was going on, he said.

The State Department hired Blackwater and Martin said it took four days for the department to arrive on the scene to look into the shootings. He said the investigation was pathetic, incomplete, haphazard and that "most of all it seemed bent on clearing the contractors."

The State Department did not immediately respond to an email request for comment late Tuesday.

One of the guards, Nicholas Slatten, is accused of first-degree murder. The other three Paul Slough, Evan Liberty and Dustin Heard are on trial for voluntary manslaughter, attempted manslaughter and gun charges.

In the aftermath of the shootings, Liberty was slapping people on the back, just as at a football game, Martin said.

The guards pleaded innocent to all charges.

On Tuesday, Martin displayed graphic photos and video of the scene in the Iraqi capital, including a picture said to be the face of a motorist prosecutors said had been shot in the head. The victim was identified by the prosecution as the first victim to be killed in the shootings, Ahmed Haithem Ahmed Al Rubia'y. Slatten is charged with first-degree murder in his death.

On Monday, in a preview of the photos and video, lawyers for the security guards had argued that it was unfair to the defendants to show the jury the photograph the prosecution said was of Al Rubia'y. They said the picture was meaningless without testimony from a doctor or a forensics expert. US district judge Royce Lamberth said the photo alone was evidence that could be shown to the jury. But Lamberth ruled out another photo showing brain matter on the street and blood spatters on a car window.

On Monday Slatten's lawyer, Thomas Connolly, said it was "pure imagination" that the photo of the car driver amounts to evidence of Slatten firing the shot that killed the driver.

Lawyers for the Blackwater defendants are focusing on self-defense and state of mind in a war zone to explain their clients' actions.

One expert witness they plan to call would testify about the use of force in combat situations and the general threat level in Baghdad at the time of the shootings.

Another expert witness will testify that certainty or absolute proof that a perceived threat was deadly or imminent was not required in order for a contractor to respond with deadly force. Rather, an individual would need only a reasonable perception, based on all the circumstances of which he was aware.

Slatten could be sentenced to life in prison if convicted. The other guards face a mandatory minimum penalty of 30 years in prison if they are convicted of the gun charge and at least one other charge.

