In calls with the police after the shooting began, he pledged his allegiance to Abu Bakr al-Baghdadi, the leader of the Islamic State group, declared himself to be an Islamic soldier and demanded that the United States stop bombing Syria and Iraq, the FBI said.
"I let you know, I'm in Orlando and I did the shootings," he said, according to a partial transcript of a 911 call made public by the FBI on Monday.
Attorney General Loretta Lynch has taken pains not to describe radical extremism as his sole motivation and declined in an interview with The Associated Press on Tuesday to rule out any other possibility, including that he was secretly gay. She also declined to say what evidence, if any, existed to support alternate theories but said investigators remain focused on why he picked a gay nightclub as the target of his attack.
Over the past two years, the Islamic State has targeted gay men for death in keeping with its radical interpretation of Islam, throwing some from tall buildings in Iraq and Syria.
In the interview and in later remarks to reporters, Lynch called the attack that killed 49 people an act of both terror and hate.
"While we know a lot more about him in terms of who he was and what he did, I do not want to definitively rule out any particular motivation here," she told the AP, later adding, "It's entirely possible that he had a singular motive. It's entirely possible that he had a dual motive."
Mateen had a wife who has been extensively interviewed by federal investigators. He also had a young son.
Jim Van Horn, 71, who said in the days after the attack that he recognized Mateen from previous visits to the Pulse, said Friday he wasn't sure why investigators wouldn't have discovered persuasive evidence of that, though he said he had no concrete evidence himself. He said he has not spoken with investigators and that they have not reached out to him.
Van Horn also said some people may be reluctant to talk about a past relationship with Mateen. "Nobody's going to say they slept with a terrorist and be on national TV," Van Horn said.
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Associated Press writer Allen Breed in Raleigh contributed to this report.
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Follow Eric Tucker on Twitter at http://www.twitter.com/etuckerAP