Contradictions and Violence on the Path to a Gay Nightclub

WASHINGTON — When a young American man from coastal Florida drove a truck packed with explosives into a hilltop restaurant in Syria in May 2014, F.B.I. agents scoured his online postings and interviewed his contacts in Florida in a scramble to determine who, if anyone, might try to launch a similar attack inside the United States.

One of the people they spoke to was Omar Mateen, a young security guard from a nearby town who had attended the same mosque as the suicide bomber and had been on a terrorism watch list for incendiary comments he once made to co-workers at a local courthouse. But the F.B.I. soon ended its examination of Mr. Mateen after finding no evidence that he posed a terrorist threat to his community.

That hopeful conclusion was upended in a bloody spasm of violence early Sunday morning when Mr. Mateen fatally shot dozens of people at a nightclub in Orlando, Fla., before being killed by police officers who stormed the club to end the standoff. The horrific events at the Pulse nightclub left 49 dead and have left family members, neighbors and federal investigators trying to piece together clues about what might have led Mr. Mateen, 29, to carry out such unspeakable violence.