Updated 4:29 p.m. ET
The shooter responsible for the deadliest mass shooting in U.S. history had pledged allegiance to the Islamic State in Iraq and Syria, or ISIS, a lawmaker said.
"According to local police, he made a pledge of allegiance to ISIL," Rep. Adam Schiff, a Democrat from California and the second-highest ranking member of the House Intelligence Committee, said on Sunday during an appearance on CNN, using another name for the terror group.
The gunman, who was shot and killed in a shootout with police, made the pledge during a 911 call in which he also referenced the Boston Marathon bombing, the news organization later reported. He was identified as Omar Mir Seddique Mateen, a 29-year-old U.S. citizen and Muslim who lived in Fort Pierce, Florida, and whose parents were of Afghan origin, Fox News reported.
It wasn't clear whether Mateen was acting as alone or coordinating with ISIS in the Middle East or abroad, Schiff said. ISIS officials have reportedly encouraged supporters to launch attacks during Ramadan, a month-long religious holiday that began last week.
The incident was the deadliest mass shooting in American history, with at least 50 individuals confirmed dead and another 53 injured. The shooting began around 2 a.m. Sunday morning at a packed Orlando nightclub called Pulse, which caters to the lesbian, bisexual, gay and transgender, or LBGT, community.
Both Florida Gov. Rick Scott and President Barack Obama described the incident as an "act of terror."
"This was an act of terror and act of hate," President Barack Obama said during a press conference at the White House.
"We've reached no definitive judgment on the precise motivations of the killer," he added. "The FBI is appropriately investigating this as an act of terrorism. And I have directed that we must spare no effort to determine what -- if any -- inspiration or association this killer may have had with terrorist groups. What's clear is that he was a person filled with hatred. Over the coming days, we will uncover why and how this happened, and we'll go wherever the facts lead us."
Obama directed flags to be flown at half-staff at the White House, public buildings and grounds, military posts and naval stations, and naval vessels in the U.S. and abroad until Thursday.
Schiff noted that ISIS frequently targets gays for horrific executions. He cited a report from earlier this year indicating a teenage boy was reportedly thrown off a roof by ISIS militants in Syria after he was accused of being gay.
The congressman said Mateen, who worked as a security guard at a British firm called G4S, was armed with an assault rifle and a handgun, and was not carrying a suicide vest as previously reported.
Mateen reportedly held dozens of people hostage until about 5 a.m., when the Orlando Police Department's SWAT team raided the building using an armored vehicle and stun grenades, and killed him, The New York Times reported. One officer was shot at, but the bullet struck his kevlar helmet -- which the department credited with saving his life.
The Orlando shooting is the deadliest terrorist attack on U.S. soil since Sept. 11, 2001, when al-Qaeda militants crashed airliners into the World Trade Center in New York City, the Pentagon near Washington, D.C., and a field in Pennsylvania, killing nearly 3,000 people.
Other terrorism-related incidents in the U.S. have occurred at Fort Hood, Texas, in 2009; the Boston Marathon in 2013; and San Bernardino, California, in 2015.
The U.S. has in recent years deployed troops back to Iraq, as well as into Syria, to help repel advances made by ISIS in those countries.
President Barack Obama has authorized the deployment of about 4,000 U.S. troops to Iraq for the campaign against ISIS. He has also approved sending a few hundred special operations forces into Iraq and Syria -- and appears poised to shelve plans to decrease the number American service members in Afghanistan from 9,800 in part because of ISIS threats.
Schiff said U.S. intelligence agencies were trying to determine whether they had any information about the shooter and his connections to radical terrorism.
"We are still looking to see whether we had any kind of advance notice that this plot may have been in the works," he told CNN's Jake Tapper. "As good as our agencies are, they're simply not able to stop a motivated, self-radicalized person who keeps the details of their plotting to themselves."
Mateen came to the attention of authorities at the FBI in 2013 and 2014, a spokesman said.
"Those interviews turned out to be inconclusive, so there was nothing to keep the investigation going," FBI Assistant Special Agent Ronald Hopper told reporters Sunday, according to CNN.
At the time of Sunday's shooting, Mateen was not under investigation or surveillance, Hopper told the news organization.