Jury selection began Wednesday in the trial against four former private security guards accused of killing Iraqi civilians in Baghdad in 2007—six years after the government first brought charges.

The trial in Washington federal court is expected to last several months. More than 100 prospective jurors were brought into the courtroom on Wednesday for vetting. U.S. District Senior Judge Royce Lamberth didn’t say how long it would take to seat a jury. The jurors were tasked Wednesday with filling out a written questionnaire; individual questioning is expected to begin Thursday.