ORLANDO, Fla. – The media has identified Omar Mateen, a 29-year-old U.S.-born citizen,  as the suspect in Sunday’s mass shooting that left at least 50 dead and more than 50 others wounded at the Pulse Nightclub, a gay nightclub in Orlando, Florida.

Authorities said Sateen had called 911 moments before to pledge allegiance to the leader of ISIS, law enforcement sources told NBC News. He appeared to be a follower of ISIS propaganda and referenced the Tsarnaev brothers, who carried out the Boston Marathon bombings in 2013, at the scene, sources said. Reports also stated that the FBI had been watching Mateen prior, and had even interviewed him at one point out of suspicion that he was an ISIS sympathizer. The incident is being called the biggest mass shooting in U.S. history. Mateen’s parents were born in Afghanistan, and he was “on the radar” of U.S. officials for some time, but was not the target of a specific investigation, law enforcement officials told ABC News.Orlando Mayor Buddy Dyer said 50 people were slain in what CNN and the Associated Press are calling the worst mass shooting in US history, early Sunday at an Orlando gay nightclub, and the shooter was killed in a gunfight with police, authorities said.

Dyer also declared a state of emergency in the city of Orlando and Gov. Rick Scott issued a state of emergency after the shooting, which police are considering an act of domestic terror.

Sen. Bill Nelson said that there appears to be connection to radical Islamic terrorism, possibly ISIS.

“I asked the FBI if there was any connection to Islamic radicalism. There appears to be. But they are naturally cautious and waiting throughout to see if if this is, facts emerge,” Nelson said at a news conference Sunday afternoon.

CBS News identified the shooter as Omar Mateen, a U.S. citizen from Port St. Lucie, and released a picture of Mateen, saying he has been on law enforcement radar in the last 5 years. The FBI says they are not identifying the shooter and that they are conducting a general investigation right now. They say they feel confident there are no other threats to the immediate area or the United States.

The shooting, which Orlando Police Department is calling “one of the worst tragedies,” unfolded at Pulse, described on its website as the “hottest gay bar in Orlando,” which is located at 1912 S. Orange Avenue near Kaley Street.

In addition to the slayings, at least 53 other victims were taken to area hospitals for medical treatment, police said. There were an estimated 320 people in the club when it opened Saturday night.

Orlando police said at their 10:30 a.m. news conference that they have been searching and investigating the suspect’s vehicle. They are also asking anyone who attended Pulse nightclub and left before the shooting to call the FBI at 1-800-CALL-FBI.

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

A clubgoer said 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 about 100 rounds were fired.

“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.

Hopper said that it’s not believed there is any other threat to the Orlando area in connection with the mass shooting.

The White House released a statement Sunday morning, reading “The President was briefed this morning by Lisa Monaco, Assistant to the President for Homeland Security and Counterterrorism, on the tragic shooting in Orlando, Florida. Our thoughts and prayers are with the families and loved ones of the victims. The President asked to receive regular updates as the FBI, and other federal officials, work with the Orlando Police to gather more information, and directed that the federal government provide any assistance necessary to pursue the investigation and support the community.”