The 14 Iraqi civilians mowed down by four guards with the Blackwater private security firm were of no threat and "were not legitimate targets," a prosecutor said Wednesday, as their high-profile trial reached its climax.

The four, who were guarding a US diplomatic convoy when they opened fire, are accused of shooting dead the unarmed Iraqis, including women and children, who were desperately trying to flee for their lives on September 16, 2007 in Baghdad's Nisour Square.

The hail of gunfire, which also wounded 18 people, exacerbated Iraqi resentment toward Americans and was seen by critics as an example of the impunity enjoyed by private security firms on the US payroll in Iraq.

At the close of the two-and-a-half-month trial in a federal court in Washington, US Attorney Anthony Asuncion asked simply what had motivated the men, now former Blackwater employees, to fire on the terrified 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 (four) 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 the 14 people killed and 18 wounded, the prosecutor asked the jury to find the four guilty.

Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten, who have pleaded not guilty, sat quietly dressed in suits and ties.

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.

Earlier in the trial, defense attorney Bill Heberlig stressed that security was facing terrible threats after the September 11 strikes on the United States.

"This was not Dupont Circle (in Washington), 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Blackwater, whose license to work in Iraq was revoked by Baghdad, was renamed Xe Services in 2009 and then Academi in 2011.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.