FILE - In this April 19, 2012 file photo, Defense Secretary Leon Panetta and Joint Chiefs Chairman Gen. Martin Dempsey confer on Capitol Hill in Washington prior to testifying. U.S. officials have settled on two main explanations for why Afghan security forces are turning their guns on their Western partners: infiltration by the Taliban and a U.S.-Afghan culture clash. Both causes suggest the problem may persist or worsen as American and other coalition forces shift further into an advisor role. (AP Photo/J. Scott Applewhite, File)