Blackwater guards fired dozens of shots into cars and people, an FBI expert testified, in an example of the brutality of the 2007 killings that left 14 Iraqis dead in Baghdad.

Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten appeared dressed in suits and ties before a federal court in Washington as the second trial in the case entered its fifth week.

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

All four have pleaded not guilty.

The Blackwater employees were guarding a US diplomatic convoy when they opened fire, killing 17 unarmed Iraqi civilians according to an Iraqi investigation, or 14, according to the US count. The hail of gunfire also wounded 18 people.

Six of the guards had started the shootings.

FBI expert Douglas Murphy said he traveled to the site of the killings twice to examine the cars involved in the shootings.

As photographs were projected in the courtroom, he spoke of "significant damage" to the 11 vehicles he observed in March 2008 and June 2009.

Murphy pointed to a white KIA riddled with bullet holes, including 29 in the front alone. There was also a Volkswagen that had 13 bullet holes on the driver's side alone.

In other cases, there was nothing left to investigate because "the entire vehicle has been burnt, the seats are no longer there, the car has no window," the expert added.

Asked about the weapons and ammunition used by the four defendants that day, Marine expert Shelby Lasater stressed the grenades used are "designed to penetrate armor and to cause casualties or kill."

"It mushrooms out, it will blow up in a circle and back," he added, noting the grenades could explode across a 540-foot (165-meter) radius.

- $1 billion after threat -

The killings exacerbated Iraqi resentment toward Americans, and were seen by critics as an example of the impunity enjoyed by private security firms on the US payroll in Iraq.

Blackwater, whose license to work in Iraq was revoked by Baghdad, was renamed Xe Services in 2009 and then Academi in 2011.

Upon President Barack Obama's arrival in office in 2009, the State Department canceled its contract with the firm.

Yet the State Department awarded more than a billion dollars in funding to Blackwater and its successor firms even after its top manager in Iraq allegedly threatened to kill a government investigator, The Huffington Post reported.

Last week, The New York Times reported that lead State Department investigator Jean Richter detailed in a memo that the Blackwater official had threatened her for probing the company's performance just weeks before the deadly shooting in Baghdad's Nisour Square.

The threat "sent a clear message that the Blackwater contractors saw themselves as 'above the law' and actually believed that 'they ran the place'," Richter said in a memo cited by the Times.