Omar Mateen | KERA News

Omar Mateen

Krystina Martinez/KERA News

The fallout from last weekend’s Orlando shootings has special resonance for Araf Hossain. He co-founded Alpha Lambda Mu, the country’s first all-Muslim fraternity, at UT-Dallas in 2012. Hossain’s now trying to launch a new project called Muslims Against ISIS.

Omar Mateen, who authorities say killed 49 people at the gay nightclub Pulse in Orlando, Fla., was reportedly no stranger to the club.

Multiple media outlets are reporting that some Pulse regulars recognized Mateen, saying that he had spent time at the nightclub before the shooting early Sunday. Witnesses have said the same to authorities, NPR's Dina Temple-Raston reports, citing U.S. officials with knowledge of the investigation.

Meanwhile, at least two men have told the media they interacted with Mateen on gay dating apps. Investigators have not commented on the claims.

Citing "strong indications of radicalization by this killer" in Sunday's mass shooting at the Pulse Orlando nightclub in Orlando, Fla., FBI Director James Comey said there are conflicting signs about the gunman's motivations — and no sign that he was acting under orders.