Family members waiting for word from the police after arriving down the street from the shooting at the Pulse gay nightclub in Orlando, Florida. AP The ISIS-affiliated news agency Amaq News broadcast a claim of responsibility from ISIS for a shooting rampage at a gay nightclub in Orlando that left 50 people dead over the weekend.

The agency said the shooting was the work of an "ISIS fighter," multiple media outlets have reported.

The gunman, identified as 29-year-old Omar Mateen, called 911 sometime Saturday night and pledged allegiance to ISIS, mentioning the Boston marathon bombers during the call.

Sen. Bill Nelson of Florida said Sunday in a news conference that Mateen, who was from Fort Pierce, Florida, had "some connection to ISIS," and Rep. Adam Schiff — the ranking Democrat on the House Intelligence Committee — said Mateen had pledged allegiance to ISIS, according to information he had received from the Department of Homeland Security.

"This is clearly an act of terror," Gov. Rick Scott of Florida said in a news conference on Sunday. President Barack Obama called the shooting an "act of terror" and "an act of hate."

Law enforcement confirmed that the incident was being treated as an act of terrorism.

Mateen was on an FBI list of suspected ISIS sympathizers and was looked into by federal authorities once in 2013 and again in 2014. He was the subject of a brief FBI investigation in 2014 before the case was closed.

"The FBI first became aware of Mateen in 2013, when he made threats to coworkers, including possible ties to terrorist activities," an FBI spokesman said Sunday in a news conference.

"In 2014, Mateen again came to the attention to the FBI because of possible ties to an American suicide bomber, Moner Abusalha. The FBI interviewed Mateen and determined that contact was minimal and did not constitute a substantive threat at the time."

Mateen legally purchased the two firearms used in the attack on the nightclub — a handgun and a long gun — in the past few days, a police spokesman said in a news conference on Sunday.

'ISIS-directed' versus 'ISIS-inspired'

Mateen's connection to ISIS does not necessarily mean he was in direct contact with the radical jihadists or that the group directed him to carry out the attack. Nelson, the Florida senator, noted that the apparent connection to ISIS was not "official," and US officials, speaking on the condition of anonymity, told Reuters they had yet to uncover direct contacts between the gunman and any extremist group.

"There is a big difference between ISIS-directed and ISIS-inspired," national-security analyst Juliette Kayyem said on CNN.

The Islamic State said in a recent message to followers that they should mount attacks during this month, which is the Islamic holy month of Ramadan.

Mir Seddique Mateen, Omar Mateen's father, told NBC News that his son's actions had "nothing to do with religion." He said his son got angry when he saw two men kissing in Miami a couple of months ago, and he thought homophobia may have inspired his son to attack the nightclub.

A photo from Omar Mateen's MySpace page. Screenshot/MySpace

"We are saying we are apologizing for the whole incident," Seddique said. "We weren't aware of any action he is taking. We are in shock like the whole country."

Seddique, an Afghan immigrant to the US, has his own YouTube channel where he has expressed his support for the Afghan Taliban, and he recently declared his candidacy for the Afghan presidency.

The YouTube videos come from a television show Seddique hosted called "Durand Jirga" on a channel called Payam-e-Afghan, which broadcasts from California, The Washington Post reported.

Chris Harmer, an analyst at the Institute for the Study of War, told Business Insider on Sunday that "at this point, it's anyone's guess as to how involved Omar Mateen was with either Al Qaeda or ISIS."

"The 911 call pledging allegiance to ISIS shows he was emotionally or intellectually sympathetic to ISIS, but does not show that he had any contact with ISIS," he said. "Bottom line, it is clear that Mateen was, at a minimum, influenced by ISIS, expressed some loyalty to ISIS, but it is not clear that he had any communication or connection with ISIS beyond that."

JM Berger, an expert on the Islamic State and coauthor of "ISIS: State of Terror," largely agreed.

"His pledge of allegiance to ISIS does not necessarily mean he was affiliated, although that may turn out to be the case. We're still very early in the investigation," Berger told Business Insider.

"Calling 911 minutes before to pledge allegiance is like what happened in San Bernardino," Mia Bloom, an ISIS expert and former fellow at the International Center for the Study of Terrorism at Penn State, told Business Insider.

FBI assistant special agent in charge Ron Hopper with law enforcement and local community leaders at a news conference on Sunday in Orlando. Gerardo Mora/Getty Images

Tashfeen Malik, the female shooter in the San Bernardino attack that left 14 people dead, pledged allegiance to ISIS leader Abu Bakr al-Baghdadi on Facebook during the rampage.

"This was an act of terror and an act of hate of the LGBT community," Bloom added. "ISIS is well known for despising and hunting down gay people, throwing them off the tallest buildings in Raqqa, etc. But we need more information to know whether there was anything more than a superficial (and last-minute) connection to ISIS."

He 'was not a stable person'

The overnight shooting at Pulse nightclub — a gay club in central Orlando — is the deadliest shooting in US history, with more fatalities than the mass shooting at Virginia Tech in 2007 (32 dead) and the mass shooting at Sandy Hook Elementary School in Newtown, Connecticut, in 2012 (27 dead).

Mateen, a US citizen, was born in New York in 1986. An FBI representative said he "was organized and well-prepared" to carry out the attack.

Mateen's ex-wife told The Washington Post that he beat her and "was not a stable person."

"He beat me," she said. "He would just come home and start beating me up because the laundry wasn't finished or something like that."

Friends and family members outside the Orlando Police Headquarters during the investigation of the shooting at Pulse. Thomson Reuters

One of Mateen's former coworkers described him as "unhinged and unstable" and said he spoke of killing people often.

But the imam at the Islamic Center of Fort Pierce, where Mateen went often to pray, told reporters in Fort Pierce that Mateen "was the most quiet guy — he never talked to anyone."

"He would come and pray and leave," Imam Shafiq Rahman said. "There was no indication at all he would do something violent.”

Rahman's son, on the other hand, said Mateen "seemed like an aggressive person."

"It was just his demeanor," he said. "He used to work out a lot.”

Mateen reportedly had a 3-year-old son whom he would take with him to the mosque.

The Islamic Center of Fort Pierce, Florida, seen in Fort Pierce on Sunday. The mosque is where Mateen worshiped. Ben Fox/AP

Mateen worked as a security guard in Fort Pierce and rented a car to drive to Orlando and carry out the attack, CNN's Evan Perez reported.

The leader of Orlando's Islamic faith community, Muhammad Musri, urged people not to rush to judgment or "sensationalize" the shooting.

"I call on my fellow faith leaders ... please pray for the victims and their families at this hour," he said during a press conference. "I want to caution anyone from rushing to judgment or sensationalizing the story, because we do not want the focus of the story to shift from what it is: a horrible tragedy. We are mourning, we are heartbroken."

Mateen entered Pulse shortly before 2 a.m. after exchanging fire with an off-duty police officer working security at the club, the police said in a news conference. He was armed with an AR-15 assault rifle and a handgun, according to law-enforcement officials' description of the incident. TMZ has reported that Mateen had a state firearms license.

Mateen barricaded himself inside the nightclub and took hostages just after 2 a.m. and was negotiating with the police when he gave them his name, according to CNN. A SWAT team stormed the club about three hours later and shot him dead.

At least 53 people were wounded and transported to local hospitals. About 320 people were inside the nightclub at the time of the shooting, according to the Orlando police. A state of emergency has been declared.

"Everyone get out of pulse and keep running," the nightclub said on its Facebook page at 2:09 a.m.

Pulse markets itself as "Orlando's premier gay nightclub." It was Latin night at the club, according to its Facebook page.