ORLANDO (CBSNewYork) — Clubgoers reported that gunman Omar Mateen had been at the Pulse nightclub in Orlando before he opened fire and killed 49 people there, a U.S. official told CBS News late Monday.

The official, who has knowledge of the situation, told CBS News that clubgoers had told investigators they had seen Mateen at Pulse before Saturday night. Clubgoers reported seeing him there repeatedly — weeks and even months before the attack.

The FBI late Monday was in the process of interviewing people who are volunteering information or who were injured in the club, the official said.

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Investigators have not corroborated the information, the FBI said.

FBI Director James Comey told reporters Monday that investigators are still trying to figure out the exact motive behind the shooting.

“We’re working hard to understand the killer and his motives and his sources of inspiration,” Comey said. “But we are highly confident that this killer was radicalized and at least, in some part, through the Internet.”

Comey said the killer self-radicalized on the Internet, and while he pledged allegiance to ISIS, he was not part of any terror network. He said the attack was not directed from overseas.