CHICAGO—During the first week of a months-long Chicago gang trial, federal prosecutors laid before jurors evidence of what they call “brazen violent murders” committed by the notorious Four Corner Hustlers, a street gang that had terrorized the West Side of Chicago for decades. Pictures of lifeless bodies were shown on the screen, blood-stained shirts and pants were brought to the courtroom, and bullets that ended lives were displayed at the witness stand. The federal judge presiding the case, Thomas Durkin, time and again reminded the jurors not to be swayed by the horrific pictures, nor to forget the defendant is presumed innocent throughout the trial. The burden is always on the prosecutors, he said, to prove charges beyond a reasonable doubt through evidence. Scores of eyewitnesses, detectives, forensic investigators, medical examiners, and a former state attorney testified what they saw or knew about the murders of Carlos Caldwell, Maximillion McDaniel, …