Four Blackwater guards accused of killing 14 Iraqi civilians turned "innocent people ... into bullet-riddled corpses" for no reason, the prosecution has told their US trial as it neared its conclusion.

A federal court in Washington heard on Wednesday the events of September 16, 2007 in Nisour Square, Baghdad, when the four men opened fire while guarding a US diplomatic convoy.

The gunfire killed 14 Iraqis and injured another 18.

At the close of the two-and-a-half-month trial, prosecutor Anthony Asuncion asked simply what had motivated Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten, to fire on the civilians.

"Why fire on so many innocent people? Why? Why shoot all of these people who are running away, who are trying to get away from these men? Why shoot women and children who are unarmed?" he asked.

"There's no reason. What they did was criminal.

"People who could laugh, who could love, were turned into bloodied, bullet-riddled corpses, people who were not legitimate targets ... who were no real threat to them."

After showing images of the faces of those killed and wounded, the prosecutor asked the jury to find the four guilty.

Slatten, 32, is charged with the first-degree murder of a civilian and faces life in prison if convicted. Slough, Liberty and Heard are accused of voluntary manslaughter of the 13 other victims. All deny the charges.

'Terrible threats'

Earlier in the trial, defence lawyer Bill Heberlig stressed that security was facing terrible threats after the September 11 strikes on the US.

"This was Baghdad, Iraq," Heberlig said, insisting Slatten "fired under control at a limited number of legitimate threats. He acted in self-defense, he committed no crime that day."

Blackwater, whose licence to work in Iraq was revoked by Baghdad, was renamed Xe Services in 2009 and then Academi in 2011.

The jury was expected to begin deliberations on Thursday.

Source: AFP