frame grab taken from web site video footage May 11, 2004, shows a man, who identified himself as Nick Berg of Philadelphia (C) seated in front of his five masked captors moments before he was executed. Al Qaeda's leader in Iraq beheaded an American civilian and vowed more killings in revenge for the abuse of Iraqi prisoners, an Islamist Web site said on Tuesday. After one of the masked men read out a statement, they pushed Berg to the floor and shouted 'God is greatest' above his screams as one of them sawed his head off with a large knife...