A top FBI official labeled antifa as the top domestic terrorist threat facing the United States, but could not answer basic details about the movement and its structure.

Michael Glasheen, a longtime FBI agent serving as operations director of the National Security Branch, called the movement the agency’s “primary concern right now" during a hearing before the House Committee on Homeland Security.

President Donald Trump said in September he was ”designating" antifa as a major terrorist organization after conservative activist Charlie Kirk was fatally shot while speaking at Utah Valley University.

Glasheen said the FBI shares the same concern as the president regarding the anti-fascist movement, known collectively as antifa.

“When you look at the data right now, you look at the domestic terrorist threat that we’re facing right now, what I see from my position is that's the most immediate violent threat that we're facing on the domestic side,” he said.

But when Rep. Bennie Thompson, D-Mississippi, the ranking chairman of the House Homeland Security committee, asked whether the group is headquartered or how many members it has, Glasheen did not have answers.

“We are building out the infrastructure right now,” Glasheen said.

“So what does that mean?” Thompson replied. “We’re trying to get the information. You said antifa is a terrorist organization. Tell us, as a committee, how did you come to that? How many members do they have in the United States, as of right now?”

Glasheen said the number is “very fluid” and that the investigation into the movement and its members is ongoing, comparing it to al-Qaeda and ISIS.

Antifa is described as individuals who “view themselves as part of a protest tradition that arcs back to opposition groups in Nazi Germany and fascist Italy prior to World War II,” according to the Congressional Research Service. The movement is “decentralized, consisting of independent, radical, like-minded groups and individuals,” CRS noted.

Glasheen testified alongside Homeland Security Secretary Kristi Noem, who was grilled by lawmakers over the Trump administration’s crackdown on illegal immigration.

Thompson continued to press Glasheen for more information on the structure of the movement, saying he doesn’t want names but just wants to know how many people have been identified to be part of antifa.

“Well, the investigations are active,” Glasheen responded, pausing before closing his mouth.

Thompson shook his head.

“Sir, you wouldn’t come to this committee and say something you can’t prove. I know you wouldn’t do that. But you did,” the congressman said, ending the exchange.

This article originally appeared on USA TODAY: FBI official says antifa is top domestic concern. He couldn’t say why.

Reporting by Rebecca Morin, USA TODAY / USA TODAY

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