All are members of the Senate Judiciary Committee.
In the letter, the senators said the FBI has created a new category — “racially-motivated violent extremism” — that “inappropriately” combines white-supremacist incidents with those involving “Black identity extremists.”
By doing so, the FBI has “shifted its approach to tracking domestic terrorism incidents to obfuscate the white supremacist threat,” the senators said.
“Given the large number of white supremacist attacks, we are deeply concerned that this reclassification downplays the significance of the white supremacist threat,” they said. “If we do not understand the scope of the problem, we cannot effectively address it.”
An FBI spokeswoman declined to comment.
The senators said the Justice Department and FBI revealed the change in a briefing last week with Judiciary Committee staffers that took place “nearly six months after the briefing was requested.”
In his campaign announcement video, former vice president Joe Biden highlighted President Trump’s “very fine people on both sides” comment about the 2017 white-nationalist rally in Charlottesville that left a counterprotester dead.
Trump responded by arguing that he was referring not to the self-professed neo-Nazi marchers but to those who had opposed the removal of a statue of the “great” Confederate general Robert E. Lee.
Trump has also previously played down the threat posed by white nationalism. After a gunman in March killed 49 Muslims in two consecutive mosque attacks in New Zealand, Trump was asked by a reporter whether he thought white nationalists were a growing threat around the world. “I don’t, really,” Trump replied. “I think it’s a small group of people that have very, very serious problems.”
Previously, the FBI used 11 categories for domestic terrorism, but the new system uses only four, according to the senators’ letter Thursday.
They said the “inherent problem” with the new approach was evident during last week’s meeting, when briefers from the Justice Department and FBI “could not say how many involved white supremacist violence, other than to acknowledge they were ‘a majority’ of the incidents” involving racially motivated violent extremism.
The senators also argued that the phrase “Black identity extremists” is “a fabricated term based on a faulty assessment of a small number of isolated incidents.”
Lawmakers and activists have previously condemned the term, with some voicing concern that it falsely labels black activists a national security threat.
Devlin Barrett contributed to this report.