An American-born man who'd pledged allegiance to ISIS gunned down 50 people early Sunday at a gay nightclub in Orlando, the deadliest mass shooting in the United States and the nation's worst terror attack since 9/11, authorities said. * The gunman, Omar Mateen, 29, of Fort Pierce, Florida, was interviewed by the FBI in 2013 and 2014 but was not found to be a threat, the FBI said. * Mateen called 911 during the attack to pledge allegiance to ISIS and mentioned the Boston Marathon bombers, according to a U.S. official. * Orlando police shot and killed Mateen.