A gay nightclub was the scene early Sunday of the worst terror attack in U.S. history since 9/11.
* 50 people were killed inside the Pulse club and at least 53 people were injured, police say.
* The gunman was Omar Mateen of Ft. Pierce, Florida, a law enforcement source told BC.
* Orlando police shot and killed Mateen
* Mateen called 911 around the time of the attack to pledge allegiance to ISIS and mention the Boston bombers, according to a U.S. official.
