The top U.S. counterterrorism official testified on Thursday that the federal government is aware of 18,000 people on the FBI terrorism watch list who were brought into the country during the Biden administration’s botched 2021 withdrawal from Afghanistan.

National Counterterrorism Center Director Joseph Kent told lawmakers on the House Homeland Security Committee that the center recently arrived at an “alarming” conclusion about who the United States let into the country four years ago.

“NCTC has identified around 18,000 known and suspected terrorists that the Biden administration let come into our country,” Kent said.

“We’ve identified 2,000 of that group … who have ties to terrorist organizations,” Kent said. “That is probably the top terrorist threat that we face right now. And that doesn’t include the individuals who came here illegally through the open border. That number alarmingly remains unknown at this time.”

During the Biden administration, more than 10 million non-U.S. citizens attempted to enter the country illegally or were denied admission at a port of entry along the southern border. Roughly half of those who attempted to walk across the border illegally were ultimately released into the U.S.

“The No. 1 threat that we have right now, in my view, is the fact that we don’t know who came into our country in the last four years of Biden’s open borders,” Kent said.

The House Homeland Security Committee held its annual hearing on worldwide threats Thursday, though much of the questioning focused on domestic threats rather than foreign concerns.

The 2021 airlift resulted in more than 80,000 people being flown out of Kabul and brought into the U.S. without being vetted. In one instance that has gained attention, the suspect behind the shooting of two National Guard members on Nov. 26 was identified as one of the Afghan evacuees.

The federal government has made several arrests since Thanksgiving of Afghan nationals who were plotting terrorist attacks in the U.S.