Orlando Shooter Allegedly Told Co-workers He Wanted to Become a Martyr and Was Investigated by the FBI Twice, but Cleared: Source

Federal officials were aware of Orlando gunman Omar Siddiqui Mateen long before he orchestrated Sunday’s mass shooting at LGBT nightclub Pulse, a FBI source tells PEOPLE.

Mateen was working as a security guard at a private company in early 2013 when his co-workers reported to law enforcement that Mateen had claimed to be a member of Hezbollah with ties to al-Qaida, and that he was “looking for an excuse to become a martyr,” the source says.

According to the FBI source, Mateen’s co-workers were interviewed by local FBI agents, who recommended that he be watched by the terrorist screening center. In the months between June and September 2013, the FBI interviewed “numerous witnesses” about Mateen and conducted physical surveillance operations on him on several occasions. Searches of other records and travel history, however, “found nothing noteworthy.”

Mateen acted alone in the Orlando shooting – the worst in America’s history – killing at least 49 people and injuring 53. The 29-year-old’s parents were from Afghanistan, but he was born in the United States. A second FBI source previously told PEOPLE that Mateen’s father had claimed in videos posted to YouTube that he supported the Taliban. President Barack Obama said on Monday during a briefing that Mateen “was inspired by various extremist information that was disseminated over the Internet.”

Although Mateen announced allegiance to ISIS, Obama said there was no evidence that the gunman was directed by the group. Instead, the president said that the attack was “an example of some kind of homegrown extremism.”

“His travel was consistent with religious pilgrimage,” the FBI source tells PEOPLE. “We asked our foreign partners for anything derogatory they had on Mateen and they came back with nothing.”

Related Video: ISIS Claims Responsibility for Attack at Orlando Gay Club, the Deadliest Mass Shooting in U.S. History

Mateen further denied the conversations his coworkers alleged in a 2013 interview, and said he had no ties to Hezbollah, al-Qaida or the Boston bomber orchestrators, Dzhokhar and Tamerlan Tsarnaev. The FBI’s investigation into Mateen was closed in March 2014, and he was removed from the Terrorist Screening Center’s official Terrorist Watchlist.

“We found no indications that Mateen was connected with anyone involved with terrorists,” the federal source says, noting that Mateen was interviewed one final time in May 2014 after Abu Salha’s suicide attack in Syria, but was once again cleared.

On the Pulse attack, the source says: “We have no indication this attack was directed by ISIL. It could have been ISIL-inspired, but even then, it is too early to say that definitively, despite the credit claimed by ISIL leaders.”

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On Sunday, Obama called the attack both “an act of terror and hate.”

The gunman’s ex-wife, Sitora Yusufiy, said during a press conference on Sunday that she believes Mateen suffered from mental illness, and alleged that he physically abused her during their four-month marriage.

“He was mentally unstable and mentally ill and he was obviously disturbed, deeply,” she said of her ex-husband, who she hadn’t seen or spoken to in at least seven years.