James Comey said it was still not clear if Mateen was part of an established extremist network or which group may have inspired him, but he told reporters: "We'll leave no stones unturned and we'll work all day and all night to understand the path to that terrible night."
Forty-nine people were killed and 53 others wounded in the shooting rampage at a gay nightclub in Orlando in the early hours of Sunday. Mateen, who pledged allegiance to the Islamic State group during the attack, was killed in a shootout with police.