Thousands in Orlando are mourning 49 people killed inside a gay nightclub as federal investigators examine possible motives for the gunman who committed the worst mass shooting in modern U.S. history.

The White House and the FBI said 29-year-old Omar Mateen, an American born Muslim, appears to be a "homegrown extremist" who had touted support not just for the Islamic State, but other radical groups that are its enemies.

Meanwhile, Mateen's ex-wife says he suffered from mental illness. And his Afghan-immigrant father says his son got angry recently about seeing two men kiss.

Meanwhile, thousands gathered Monday night in downtown Orlando for a vigil to support victims and survivors of the Pulse nightclub shooting on the lawn of Orlando's main performing arts venue.