text: FILE - This Aug. 20, 2014 file-pool photo shows Attorney General Eric Holder talking with Capt. Ron Johnson of the Missouri State Highway Patrol at Drake's Place Restaurant in Florrissant, Mo. As the Justice Department probes the police shooting of an unarmed 18-year-old in Ferguson, Missouri, history suggests theres no guarantee of a criminal prosecution, let alone a conviction. Federal authorities investigating possible civil rights violations in the Aug. 9 death of Michael Brown must meet a difficult standard of proof, a challenge that has complicated the path to prosecution in past police shootings. (AP Photo/Pablo Martinez Monsivais, File-Pool