Flags at the White House were lowered to half staff Sunday in mourning for what President Barack Obama called a "horrific massacre" in Orlando, Florida, where a gunman killed 50 people and wounded 53 in an attack on a gay nightclub.
Obama noted it was "the most deadly shooting in American history."
"This is a sobering reminder," he said, "that attacks on any American, regardless of race, ethnicity, religion or sexual orientation, is an attack on all of us."
The suspected gunman, a US-born Afghan-American, had been interviewed by federal agents three times since 2013 over suspected links to Islamist militants, the FBI said Sunday.
There was not yet a "definitive judgement" about the motives of the attacker, Obama said.
"What is clear is that he was a person filled with hatred," Obama said said. "Over the coming days, we'll uncover why and how this happened, and we will go wherever the facts lead us."
The president met Sunday with top advisors for homeland and national security.
"We know enough to say that this was an act of terror and an act of hate," Obama said.