EL PASO, Texas — Two U.S. Border Patrol agents were watching the Mexican boundary last year when they stopped a van carrying 743 pounds of marijuana. The driver fled back across the Rio Grande — with a gunshot wound in his buttocks. Federal prosecutors convinced a jury in March that the agents had shot a defenseless man and schemed to cover it up. Much of the evidence against them came from the drug runner, Osvaldo Aldrete-Davila, who reported the shooting to a friend at the Border Patrol in Arizona. Aldrete-Davila was given immunity from prosecution by the U.S. attorney's office....