Nearly three years later, this city on Tuesday began to relive one of the most notorious crimes in its recent history, as the first of six defendants went on trial in the execution-style murders of three students in a schoolyard on a summer night. --snip-- The murders brought the first reports to many people here of MS-13, or Mara Salvatrucha, the violent street gang that the authorities say Mr. Godinez, a native of Nicaragua, belonged to.