UPDATE 11 a.m. ET: The gunman has been identified as Omar Saddiqi Mateen, who had resided in Fort Pierce, Florida. This shooting is now the most deadly mass shooting in U.S. history.

Orlando Mayor Buddy Dyer says that "only a handful" of the victims have been identified thus far. Pulse is a gay nightclub, which may have factored into why Mateen picked it as a target.

Neither ISIS nor al Qaeda has claimed credit for the rampage, though officials do feel that Mateen was a terrorist sympathizer. It is not yet clear whether he had any direct contact with terrorist leaders, either at home or abroad.

UPDATE 4 p.m. ET: Mateen, the shooter, called 911 before the attack to pledge his allegiance to ISIS. He also mentioned the Boston Marathon bombers. President Obama stated in an address that this was "an attack on all of us." In addition, a suspect was stopped in Los Angeles who had explosive powder, a camouflauge outfit and assault weapons. He said he was on his way to the Los Angeles Pride festival.

UPDATE 5:50 p.m. ET: Orlando officials have begun listing names of the deceased on their website.

Original story begins here:

At about 2 a.m. Sunday morning, a gunman opened fire inside the Pulse nightclub in Orlando, Florida, killing approximately 50 people and injuring at least 53 more (reports initially said 20 dead). A police officer working at the club exchanged fire with the suspect, who took hostages into a bathroom at around 5 a.m. Soon after, police killed the suspect. A spokesman for the Florida Department of Law Enforcement said the incident was being officially classified as an "act of terrorism."

The shooter's name has not been revealed, though officials say he was not from the Orlando area. He was carrying an assault rifle and a handgun, as well as a possible explosive device. The FBI is also on the scene, investigating whether there may be ties to terrorism.

Anyone with any information that may aid the investigation should call the FBI hotline at 1-800-CALL-FBI.

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