Officials asked the public to help in the investigation of Sunday's carnage at the Pulse nightclub in Orlando -- the worst mass shooting in American history, which left 49 people dead and 53 others wounded.
Gunman Omar Mateen was killed in a shootout with police.
Authorities refused to comment on reports that Mateen's wife would face charges over her alleged knowledge of his intentions to carry out an attack, calling any talk of charges "premature" and saying no stone would be left unturned in the probe.
"Civil rights violations are a priority for the FBI," assistant special agent Ron Hopper told reporters. "We will investigate reported incidents against individuals based upon any class, any protected class, to include race, religion, and sexual orientation."
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US attorney Lee Bentley chimed in: "Making these threats is not only wrong, in most cases, making these threats is illegal. Stop it. Any threats like this detract from what we're doing in law enforcement."
"We're scared," Bedar Bakht, a taciturn Pakistani in his 50s who worships at the same mosque attended by Mateen, told AFP.
Mateen's motives for carrying out the slaughter are still unclear.
In a 911 call during the attack in the early hours of Sunday, he pledged allegiance to the leader of the Islamic State group, and President Barack Obama has said he was radicalized by reading extremist propaganda online.
But witnesses also say he was a regular at the Pulse club, and was using gay dating apps.