A top FBI official called the anti-fascist movement antifa the biggest domestic terrorism threat to the U.S. during a congressional hearing on Thursday, but struggled to answer detailed questions about the unstructured far-left campaign.

Michael Glasheen, operations director of the FBI’s National Security Branch, said antifa was the agency’s “primary concern” and “the most immediate violent threat that we’re facing.”

Glasheen did not answer a question from the top Democrat at the hearing, U.S. Representative Bennie Thompson, about the group’s location. When asked about the number of members, Glasheen said it was “very fluid” and that “investigations are active.”

“Sir, you wouldn’t come to this committee to say something that you can’t prove,” Thompson said to Glasheen. “I know you wouldn’t do that. But you did.”

The FBI did not immediately respond to a request for comment.

President Donald Trump, a Republican, in September designated antifa as a domestic terrorist organization although some extremism experts have argued that antifa is better viewed as an anti-fascist, anti-white supremacy ideology than a hierarchical entity.

The exchange underscored the partisan divide at the annual hearing on “worldwide threats” before the U.S. House of Representatives Homeland Security Committee. Glasheen testified alongside Joe Kent, director of the National Counterterrorism Center and Homeland Security Secretary Kristi Noem.

The FBI director typically testifies at the annual hearing but Director Kash Patel did not attend.

Glasheen said later in the hearing that the FBI had 70 active antifa investigations and related arrests were up 171% this year.

Noem called the defense of people who break immigration laws “shameful” and said lawmakers should change U.S. laws if they don’t like them.

“That’s your job,” she said. “So you should all be fired, in my opinion.”

Noem criticized vetting standards under former President Joe Biden for failing to flag an Afghan immigrant accused of attacking National Guard members in Washington, D.C., last month, even though the suspect was granted asylum in April, while Trump was in office.

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(Reporting by Jana Winter and Ted Hesson; Additional reporting by Jonathan Landay; Editing by Scott Malone)