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

Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten appeared before a federal court for the start of jury selection, which will take place over the next few days.

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

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

In a brief proceeding, the judge instructed the prospective jurors not to read or watch anything about the case, specifically mentioning social media. No text messaging about the case. No Twitter.

"Just don't do it," he said. "Just stick to the evidence" in the courtroom.

Lamberth gave no indication to the prospective jurors that the guard who faces the most serious charge is asking that he be tried separately from the other three.

The Justice Department opposes the request by Slatten, who is charged with first-degree murder. Details of the request and the prosecutors' response are sealed from public view for now. In a court filing, federal prosecutors referred to the guard's request to sever his case.

The other three guards — Slough, Liberty and Heard — are charged with manslaughter, attempted manslaughter and gun charges. All four have pleaded not guilty.

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

The killing exacerbated Iraqi resentment toward Americans, and critics saw it as an example of the impunity enjoyed by private security firms on the U.S. payroll.

Slatten could be sentenced to life in prison if convicted of murder, while the others face a mandatory minimum penalty of 30 years if convicted of the gun charge and at least one other charge.

Wire services