UPDATE: 16 hours after Omar Mateen opened fire inside a crowded Orlando nightclub, 50 have been confirmed dead and at least as many injured in the deadliest mass shooting in U.S. history. 
Millions have taken to social media to share their grief and disbelief over the shooting, which Orlando police and the FBI say was premeditated after Mateen, a U.S. citizen born in New York, drove from Miami with a car full of weapons. The gun he used to shoot up Pulse nightclub and kill dozens of innocent people was legally obtained, even though Mateen had been interviewed by the FBI in 2013. According to CNN, Mateen pledged allegience to ISIS on a 911 call during the attack, and referenced the Boston marathon bombers. 
President Obama addressed the nation soon after the attack, sharing his condolences to the bereaved and reassuring scared citizens across the country:

PREVIOUS: ABC News reports that the death toll has climbed to at least 50 in the mass shooting at Pulse Nightclub in Orlando, Florida. Authorities are classifying the incident as domestic terrorism and the FBI are involved. The shooter, identified as U.S. citizen Omar Mateen, has been killed by police.

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Shocking and horrifying news out of Orlando, Florida.
In the wee hours of Sunday morning, a gunman opened fire inside of a gay nightclub in the Sunshine State, killing at least 20 people before he was shot dead by police. At least 42 more people have been transported to local hospitals for medical treatment.
The incident took place around 2AM at Pulse Nightclub in Orlando, Orlando Police Chief John Mina said Sunday morning, just hours after a shooter opened fire in the club, adding that police have shot and killed the gunman, who has been identified as Omar Mateen, a U.S. citizen. 

"It's appears he was organized and well-prepared," the chief said, adding that the shooter had an assault-type weapon, a handgun and "some type of (other) device on him." Law enforcement sources told CNN that the device, which was strapped to the suspect, was possibly explosive, but authorities don't know if it was real or not. A canine unit indicated there were explosives inside the suspect's car as well, the sources said.
The shooting began as a shootout between Mateen and a security officer working at the club outside the doors, authorities say, before Mateen ran inside the club and it turned into a "hostage situation." 
According to CNN, Orlando authorities said they consider the violence an act of domestic terror and the FBI is involved. While investigators are exploring all angles, the FBI says they "have suggestions the individual has leanings towards (Islamic terrorism), but right now we can't say definitely."
The suspect is not from Orlando, authorities add. It also hasn't been determined if homophobia is a factor, though the facts that the shooting took place at a gay nightclub during LGBT Pride Month are indications that it may have been.