ORLANDO -- The Islamic State of Iraq and Syria (ISIS) has claimed responsibility for the attack early Sunday morning at a gay nightclub in Orlando, Florida that left at least 50 dead and 53 injured.

The group's official radio station, al-Bayan, said Monday morning that, "Over 100 Crusaders killed and injured after an attack on their gathering at a night club in America."

"We begin from America, where Allah has enabled brother Omar Mateen, one of the soldiers of the Caliphate in America, to carry out a raid where he was able to infiltrate a Crusaders' gathering at a gay night club in Orlando, Florida. Allah enabled him to inflict heavy casualties amongst the filthy Crusaders. He killed and injured over a hundred of them. This is the biggest raid to be carried in America after the raid of Manhattan 16 years ago. All praise to Allah."

ISIS has in the past claimed responsibility for attacks that did not originate within their command structure or territory, and analysts say this latest claim still does not prove ISIS was directly involved in the shooting.

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The gunman behind the mass shooting has been identified as Port St. Lucie resident Omar Mateen, CBS News confirms. Officials are still investigating the exact motives for the shooting, and both ties to radical Islam as well as a general hate crime motivation are being considered.

A U.S. intel source tells CBS News that Mateen called 911 during the shooting from the bathroom. They called him back and that is when Mateen pledged alliance to ISIS and al Baghdadi.

The source tells CBS News that Mateen made two foreign trips, one in 2011 to Dubai and one to Saudi Arabia in 2012.

He was not on a current watch list. He was on the Terrorist Screening database when the FBI was questioning him. The source tells CBS News he was removed when the FBI closed their investigation.

The 2013 investigation began when Mateen told co-workers and others he knew the Tsarnaev brothers who were responsible for the Boston marathon bombings.

The FBI confirmed at a press conference Sunday afternoon that federal authorities twice looked into Mateen in the past.

In 2014, Mateen was questioned because of what FBI Special Agent Ron Hopper called a possible connection to a suicide bomber

Omar Mateen, the 29-year-old man who authorities suspect killed 50 and injured 53 in a mass shooting at Pulse, a gay nightclub in Orlando, on Sunday, July 12, 2016. CBS

Hopper said agents contacted Mateen three times, but ultimately determined there was no reason to pursue an investigation.

"Those interviews turned out to be inconclusive so there was nothing to keep the investigation going," Hopper said.

Mateen's ex-wife told The Washington Post that she met him online about eight years ago and decided to move to Florida and marry him. The ex-wife, who wasn't named in the report, says at first the marriage was normal, but then he became abusive.

They were together for only a few months and her parents intervened when they learned Mateen had assaulted her. She says he wasn't very religious and gave no signs of radical Islam.

She said he owned a small-caliber handgun and worked as a guard at a nearby facility for juvenile delinquents.

Mateen's ex-wife said his family was from Afghanistan, but her ex-husband was born in New York. His family later moved to Florida.