Former Blackwater Worldwide guard Evan Liberty, right, arrives at federal in Washington. After years of delays, four former guards from the security firm Blackwater Worldwide are facing trial in the killings of 14 Iraqi civilians and the wounding of 18 others in bloodshed that inflamed anti-American sentiment around the globe. (AP Photo/Cliff Owen)

Washington - Four former employees of the notorious Blackwater security firm went on trial here Wednesday, seven years after allegedly killing at least 14 Iraqi civilians in Baghdad.

Backed by an army of lawyers, Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten appeared before a federal court for the start of jury selection.

The trial was expected to last between five and six months, Slatten's lawyer, Thomas Connolly, told AFP.

Judge Royce Lamberth said “a very high number of Iraqi witnesses” are scheduled to testify about what they saw on September 16, 2007 in Baghdad's Nisour Square.

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.

The killing 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.

Slatten is charged with the first degree murder of a civilian. Slough, Liberty and Heard are accused of voluntary manslaughter of the 13 other victims.

All four have pleaded not guilty.

Their trial follows a tortuous legal process as the case slowly wound through US courts.

In 2009, a US judge dismissed charges against five former Blackwater employees because certain statements they made immediately after the event could not be used against them.

Two years later, an appeals court reinstated the indictments against four defendants, opening the way for the current trial.

But a judge dismissed the case against Slatten in April because of a technicality. Federal prosecutors then re-filed first degree murder charges against him several weeks later.

Slatten faces a maximum penalty of life in prison, if convicted, but prosecutors would have to convince a jury that he acted with premeditation.