Breaking: Mass Casualties at Orlando Gay Nightclub, Shooter Dead

Police have shot and killed the gunman, who opened fire in Pulse nightclub, a gay club in Orlando, Fla., overnight, say Orlando police. Police Chief John Mina told reporters. "It's appears he was organized and well-prepared," the chief said. The shooter had an assault-type weapon and a handgun.

Multiple outlets are reporting that a shootout with three police officers began at 2:02 a.m. outside the nightclub, then the shooter ran inside. "That turned into a hostage situation," according to Mina. A SWAT team was called in. Patrons inside the club sent updates to police, and police decided to raid the club approximately 5 a.m.

CNN reports: "More police rushed to the scene and broke down a door with an armored vehicle. At that point police shot and killed the gunman."

USA Today reports: "FBI Special Agent Ronald Hopper said the case was being investigated as a possible act of terrorism, either domestic or international. It was not clear if the shooter acted alone, he said. He said they were trying to determine if there was a connection with radical Islam."

A Facebook massage posted by Pulse read, "Everyone get out of Pulse and keep running."

More than 300 people were inside the club at the time Approximately 20 people were killed. 42 were transported for medical care, according to Mina. Police say the shooting wasn't from the Orlando area, but they have not identified the shooter.

Orlando authorities consider the shooting an act of domestic terror. The FBI is involved.

CNN has witness accounts here.