Photos: 50 Killed in Florida Nightclub Terror Attack

A gay nightclub here 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 CNN.

* Orlando police shot and killed Mateen.

"It's appears he was organized and well-prepared," Orlando Police Chief John Mina said early Sunday. The shooter had an assault-type weapon, a handgun and "some type of (other) device on him."

Officials, aided by the FBI, warned that a lengthy investigation was ahead, given the number of victims and the scope of the violence.

"There's blood everywhere," U.S. Rep. Alan Grayson told reporters. "There's an enormous amount of evidence to be collected."

It's just shocking," said Christopher Hansen, who was inside Pulse when he heard gunshots, "just one after another after another."

"It could have lasted a whole song," he said.

Hansen was getting a drink at the bar about 2 a.m. when he "just saw bodies going down."

Orlando Mayor Buddy Dyer said he had declared a state of emergency for the city and has requested the governor do the same for the state.