FBI investigators so far have not been able to substantiate claims that Orlando gunman Omar Mateen was gay or pursuing gay relationships, a senior federal law enforcement source confirmed to CBS News.

The FBI began looking into that possibility after media reports last week quoted men as saying that Mateen had reached out to them on gay dating apps and had frequented the gay nightclub where the June 12 massacre took place. One man claimed to be Mateen's gay lover in an interview with Univision that aired this week, while another recalled Mateen as a regular at the Pulse club who tried to pick up men.

But the senior federal law enforcement source said the FBI, which has conducted interviews and is reviewing evidence collected from Mateen's phone, has not found concrete evidence to corroborate such accounts nearly two weeks into the investigation.

None of the dating apps on which men have claimed to have met Mateen were found on his devices, the source said. Also, there was no information to indicate he was on any dating sites at all, the source said.

Law enforcement officials have said there is no doubt that Mateen was radicalized at some point before the Pulse nightclub attack, though there is no evidence that he was directed by any foreign terror groups.

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In calls with the police after the shooting began, he pledged his allegiance to Abu Bakr al-Baghdadi, the leader of the Islamic State of Iraq and Syria, or ISIS, 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 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, ISIS has targeted gay men for death in keeping with its radical interpretation of Islam, throwing dozens of them 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 3-year-old 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.