The gunman who fatally shot at least 50 people in a downtown Orlando nightclub has been identified as Omar Mateen, 29, of Port St. Lucie, authorities said Sunday.

As the country awoke today to the shock of yet another mass shooting, state and federal authorities were scrambling to investigate Mateen, of Afghan descent, for links to international terrorism.

Federal agents with South Florida's Joint Terrorism Task Force have joined with counterparts in Central Florida to investigate Mateen. Authorities with knowledge of the investigation say agents have already begun interviewing Mateen's relatives, and search warrants are expected to be executed this afternoon.

Investigators are exploring whether Mateen could have been self-radicalized, or had connections to overseas terrorism groups.

Police in Orlando say Mateen took scores of people hostage overnight inside Orlando's Pulse nightclub between 2. and 5 a.m., local time, Police Chief John Mina said at least 42 people were taken to local hospitals.

Heavily armed Orlando police officers stormed the nightclub, killing the man just before 6 a.m. He was carrying an assault rifle, a handgun and possibly wearing an explosive "device," police said.

The Orange County Sheriff's Office hazardous device team is searching the area.

About 320 people were in the club when the hostage situation occurred. About 30 people "were saved during the rescue," Mina said during an early morning press conference. Nine Orlando Police Department officers were involved in the shootout, and one was shot, but is alive.

More than 100 law enforcement agents, including from the FBI, are assisting in the investigation. FBI agent Ron Hopper said authorities cannot rule this out as an act of domestic terrorism, and the suspect, who is not from Orlando, may have had leanings toward extreme Islamic ideologies.

"Tonight we had a crime that will have a lasting effect on our community," said Orlando Mayor Buddy Dyer at the news conference. "We need to stand strong, we need to be supportive of the victims and their families."

Our community experienced a horrific crime today. Our hearts & prayers are with the victims and their families.

Club-goer Rob Rick told The Associated Press estimated it happened just before closing time.

"Everybody was drinking their last sip," he said.