Photo: AP

Just before he walked into Pulse, a gay nightclub in Orlando, early Sunday morning and opened fire, killing 50, Omar Mir Seddique Mateen called 911 to pledge allegiance to the leader of ISIS, Abu Bakr al-Baghdadi, also referencing the Tsarnaev brothers. But his extremism was apparently already known—the FBI had previously investigated him on at least two separate occasions.

“He’s a known quantity,” an unnamed senior law enforcement source told The Daily Beast. “He’s been on the radar before.” CBS News also reports that Mateen became a person of interest to the FBI “within the last five years.”

Born “Omar Mir Seddique” in New York, court records show that Mateen, an American citizen living in Port St. Lucie, Florida, adopted his new surname in 2006. Mateen worked as a security guard and had a Class G gun license, permitting him to carry a handgun. He was also carrying an AR-15 with high-capacity magazines. He was killed after a gunfight with SWAT officers inside the club.

At a press conference on Sunday afternoon, special agent in charge Ronald Hopper confirmed that the FBI first became aware of Mateen in 2013, after he made inflammatory comments to coworkers. He was investigated again in 2014, after he was linked to a suicide bomber.

“We know enough to say this is an act of terror and an act of hate,” President Barack Obama said at a news conference Sunday. According to the New York Times, federal law enforcement officials have not yet found any direct link between the shooter and a wider terrorist network.

Ronald Hopper, an assistant agent in charge of the F.B.I.’s Tampa Division, raised the possibility that the killer was an Islamist radical, and law enforcement officials said they were investigating the massacre as a terrorist attack. The F.B.I. set up a hotline for tips.

“We do have suggestions that that individual may have leanings towards that, that particular ideology,” Agent Hopper said at a news conference. “But right now we can’t say definitively, so we’re still running everything around.”

Representative Adam B. Schiff, a California Democrat who is a member of the House Intelligence Committee, issued a statement saying law enforcement officials told him that the gunman had pledged his allegiance to the Islamic State.

The Islamic State’s official news outlet claimed responsibility for the attack, Times reporter Rukmini Callimachi reported on twitter.

Last month, ISIS encouraged militants to strike during the holy month of Ramadan. “Make it, Allah permitting, a month of hurt on the infidels everywhere,” spokesman Abu Muhammad al-Adnani said. Al-Adnani emphasized the vulnerability of so-called soft targets (like nightclubs), and reassured militants that civilians in the West are not innocent.

“We condemn this monstrous attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured,” Rasha Mubarak, the Orlando regional coordinator of the Council on American-Islamic Relations, said in a statement. “The Muslim community joins our fellow Americans in repudiating anyone or any group that would claim to justify or excuse such an appalling act of violence.”

The shooting is the deadliest in United States history.