Authorities investigating Omar Mateen's massacre of at least 50 people at a popular gay nightclub in Orlando, Florida are trying to determine whether terrorism, homophobia or both pushed him over the edge. (NBC News)
Law enforcement sources told NBC News Mateen swore allegiance to the leader of ISIS in a 911 call moments before Sunday's rampage, though he does not appear to have direct ties to the radical group. The 29-year-old was killed in a shootout with police.
Despite being interviewed twice by federal authorities in 2013 over terror comments, Mateen had security and firearm licenses, and the weapons used in the assault were legally purchased within the past week.
Republican Donald Trump faces high stakes Monday, with a much-touted speech criticizing Democratic presidential rival Hillary Clinton that will also serve as his first public address since the Orlando attack. (NBC News)
Reacting to the tragedy, Trump tweeted acknowledgement to supporters who credited him with being right on "Islamic terrorism." Meanwhile, Clinton called for unity and more gun control. (CNBC)