Orlando, Florida (CNN)Approximately 20 people are dead inside Pulse nightclub, Orlando Police Chief John Mina said Sunday morning, just hours after a shooter opened fire in the club. At least 42 people have been transported for medical treatment, he said.
Police have shot and killed the gunman, Mina told reporters.
"It's appears he was organized and well-prepared," the chief said, adding that he had an assault-type weapon and a handgun.
Orlando authorities said they consider the violence an act of domestic terror. The FBI is involved.
The shooting began around 2 a.m., and an officer responded, Mina said. The officer engaged in a shoot out outside the club. The gunman then ran into the club.
"That turned into a hostage situation," Mina said. Authorities were getting calls from people inside the club but away from the gunman, the chief said.

More police rushed to the scene and broke down a door with an armored vehicle. At that point police shot and killed the gunman, Mina said.
CNN affiliate WKMG video captured club goers carrying injured people from the club.
"It's just shocking," said Christopher Hansen who was inside Pulse. He heard gunshots, "just one after another after another."
"It could have lasted a whole song," he said.
"Everyone get out of Pulse and keep running," the nightclub posted on its Facebook page shortly after the violence began.
Orlando police warned residents to stay away from the area, and urged people not to call their offices. They declined to provide the number of casualties, saying details will be available later. Bomb sniffing dogs are on the scene.
http://edition.cnn.com/2016/06/12/us/orlando-nightclub-shooting/index.html