Twelve hours after the attack, ISIS claimed responsibility, through a statement on an encrypted phone app, according to The New York Times. NBC News reported that Mateen had been on some sort of watch list at some point in the past - and also that Mateen called 911 right before the attack and pledged allegiance to ISIS. Steve Nesius/ReutersFBI officials said the gunman may have extremist leanings - but it's too soon to know for certain.
