ORLANDO, Florida - Fifty people were slain in a mass shooting early Sunday at an Orlando nightclub, and the shooter was killed in a gunfight with police, authorities said. Fifty-three people were wounded.

The Orlando mayor announced the new number of casualties after law enforcement was able to clear the nightclub. Chief of Police John Mina said many of the slain were found inside.

CBS News identified the shooter as Omar Mateen, a U.S. citizen from Port St. Lucie. Sources have confirmed to CBS News that the FBI is investigating whether the suspect Omar Mateen had ties to Islamic terrorism.

Pulse is described on its website as the "hottest gay bar in Orlando." The club is located at 1912 S. Orange Avenue near Kaley Street. The FBI asked any witnesses to come forward, even if they left the club before the shooting took place.

Police are posted outside Mateen's neighborhood and bomb squad investigated a vehicle and backpack outside the nightclub Sunday morning, according to News 6 WKMG.

The White House said Sunday, several hours after the incident, that President Barack Obama has been briefed and has asked for regular updates as the FBI and other federal officials work with Orlando police on the case.

Press secretary Josh Earnest said in a statement that the president has directed federal officials to provide "any necessary assistance to pursue the investigation and support the community."

There were an estimated 320 people in the club when it opened Saturday night.

"Everybody was just running out through the back (of the club)," a witness told News 6.

A clubgoer told News 6 that he saw one person holding what he called an assault rifle and firing rounds inside Pulse. Police said the gunman had an assault-type rifle, a handgun and some sort of device strapped to himself.

Another clubgoer said he heard at least 40 shots fired. Another witness said he heard about 100 rounds.

"I saw blood everywhere," another clubgoer said.

Orlando Police Chief John Mina said an officer in full uniform was working extra duty at the club and heard reports of shots fired. He engaged the shooter, who went back inside the club and created a hostage situation, Mina said.

About three hours later, Mina said a decision was made to rescue the hostages, and the shooter was killed in a gunfight with officers.

There were nine police officers involved in the shooting, one of whom was injured.

"His Kevlar helmet saved his life," said Mina, adding that the officer's injuries were not serious.

An FBI spokesman said the mass shooting is being investigated as an act of terrorism. He said authorities are looking into whether this was an act of domestic or international terror, and if the shooter was a lone wolf.

"We're looking into all angles," said Ron Hopper, who added that the shooter may have had radical Islam leanings.

Numerous emergency vehicles were at the scene, and several victims were taken to nearby Orlando Regional Medical Center. Video also showed clubgoers carrying wounded victims to the hospital.

Social media erupted with reports about the shooting, saying there were mass casualties, which Orlando police later confirmed.