A claim made by Islamic State's Amaq news agency that the militant group was behind a shooting in Orlando, Florida, that left 50 dead has not yet confirmed, Florida Senator Bill Nelson said.
"The Islamic State's news agency has just issued a statement that says that they are responsible. That has not been confirmed," Nelson told reporters in Orlando. "We'll have to see what those connections are once we get the details."
Earlier, an AIslamic State-affiliated twitter account has posted a photo of Omar Mateen, the alleged shooter who killed 50 people and injured 53 in the massacre at a gay nightclub early this morning.
A senior FBI official said Mateen might have had leanings towards Islamic State militants while a top U.S. congressman said Mateen may have pledged allegiance to the Islamic State militant group. Officials cautioned that the Islamist tie required further investigation.
Police killed Mateen after a hostage standoff.
Orlando Mayor Buddy Dyer said this was the deadliest single U.S. shooting incident, eclipsing the 32 people killed in the 2007 massacre at Virginia Tech university.
"Today we're dealing with something that we never imagined and is unimaginable," Dyer said, more than doubling an earlier estimate that about 20 bodies had been found.
A police officer working as a security guard inside the Pulse nightclub, which has operated in downtown Orlando since 2004 and was hosting some 350 revelers, exchanged fire with the suspect at about 2 a.m. (8am in Malta), police officials said.
"Everyone get out of pulse and keep running," the club's management wrote on Facebook as the incident unfolded.
A hostage situation developed, and three hours later SWAT team officers used armored cars to storm the club before shooting dead the gunman. It was unclear when the victims were killed.
Dozens of terrified patrons, some of whom had been hiding in restrooms, were rescued. One officer was injured when he was hit in his helmet while exchanging fire with the gunman, police said.
Representative Adam Schiff, the top Democrat on a congressional intelligence committee, noted that the shooting took place during Ramadan, and that ISIS leaders in Syria have urged attacks during this time.
"The target was an LGBT nightclub during Pride, and - if accurate - that according to local law enforcement the shooter declared his allegiance to ISIS, (that) indicates an ISIS-inspired act of terrorism," Schiff said in a statement.
'DIFFERENT KIND OF CASE'
If confirmed as an act of terrorism, it would stand as the deadliest such attack on U.S. soil since Sept. 11, 2001, when al Qaeda-trained hijackers crashed jetliners into New York's World Trade Center, the Pentagon and a field in Pennsylvania, killing some 3,000 people.
A pair of ethnic Chechen brothers killed three people and injured more than 260 with a pair of homemade pressure-cooker bombs at the Boston Marathon in April 2013
Asked if the FBI suspected the gunman might have had inclinations toward militant Islam, including a possible sympathy for Islamic State, Ronald Hopper, an assistant FBI agent in charge, told reporters: "We do have suggestions that the individual may have leanings toward that particular ideology. But right now we can't say definitively."
U.S. Senator Marco Rubio of Florida, who sits on the Senate intelligence and foreign relations committees, told CNN he understood that the gunman had worked for a security company and so would have undergone some background checks.
"Over the next couple of days they're going to be looking to see where this individual was inspired to carry out this horrifying act of terrorism," Rubio said. "I think we're going to be talking about a very different kind of case here soon."
President Barack Obama ordered the federal government to provide any assistance needed to Florida police investigating the shooting.
The attacker was carrying an assault rifle and a handgun, Orange County Sheriff Jerry Demings said. He was also carrying an unidentified "device", Orlando Police Chief John Mina said earlier.
Video footage showed police officers and civilians carrying some people away from the club and bending over others on the ground. Dozens of police cruisers, ambulances and other emergency vehicles could be seen in the area.
"It's definitely a tragedy not only for the city but for our entire nation," Mina told reporters. "We believe this is a tragedy that could happen anywhere in the United States of America, and what we need to do is continue to be vigilant."
Pulse, which was crowded for a Latin music night, describes itself on its website as more than "just another gay club." One of the founders and owners, Barbara Poma, opened it more than a decade ago in an effort to keep alive the spirit of her brother, who died after battling HIV.
The choice of target was especially heart-wrenching for members of the U.S. lesbian, gay, bisexual and transgender community, LGBT advocacy group Equality Florida said in a statement.
"Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety," the group said. "We will await the details in tears of sadness and anger."
Orlando has a population of 270,930 and is the home of the famed Disney World amusement park and many other tourist attractions that attracted 62 million visitors in 2014.
It was the second deadly shooting at an Orlando night spot in as many nights. Late Friday, a man thought to be a deranged fan fatally shot singer Christina Grimmie, a former contestant on "The Voice", as she was signing autographs after a concert.