The terror attack on an Orlando, Florida nightclub was the worst mass shooting in U.S. history. The shooter left 49 club-goers dead. The gunman killed by police was 29-year-old Omar Mateen. He was an American citizen who worked as a security guard and had a firearms license. FBI Director James Comey says he espoused support for a number of often conflicting Islamic organizations. To help make sense of the mass shooting, Professor Jonathan White, a terrorism expert and executive director of the Homeland Defense Initiative at Grand Valley State University held a news conference. First, his reaction.