By Mark Hosenball WASHINGTON (Reuters) - U.S. government agencies were examining social media postings purporting to show how an American who was fighting with an Al Qaeda affiliate blew himself up recently in a suicide attack in **rthern Syria, U.S. officials said. According to internet postings, the suicide bomber, who called himself "Abu Hurayra al-Amriki" carried out one of four suicide bombings on May 25 in Syria's Idlib province on behalf of Jabhat al-Nusra, al Qaeda's affiliate fighting to oust the government of President Bashar al Assad. If his identity is confirmed, he would be the first American k**wn to have committed a suicide bombing in Syria on behalf of al Qaeda, said Laith Alkhouri, a senior analyst with Flashpoint Global Partners, which monitors militant websites for government and private clients.