The rapid changes in Afghanistan are forcing the Biden administration to confront the prospect of a resurgent al-Qaida, the group that attacked America on 11 September 2001, at the same time the U.S. is trying to stanch violent extremism at home and cyberattacks from Russia and China.

With the rapid withdrawal of U.S. forces and rise of the Taliban in Afghanistan, “I think al-Qaida has an opportunity, and they’re going to take advantage of that opportunity,” says Chris Costa, who was senior director for counterterrorism in the Trump administration.

“This is a galvanizing event for jihadists everywhere.”

Al-Qaida’s ranks have been significantly diminished by 20 years of war in Afghanistan. But a June report from the UN Security Council said the group’s senior leadership remains present inside Afghanistan, along with hundreds of armed operatives. It noted that the Taliban, who sheltered al-Qaida fighters before the 11 September attacks, “remain close, based on friendship, a history of shared struggle, ideological sympathy and intermarriage.”

Pentagon spokesman John Kirby acknowledged Friday that al-Qaida remains a presence in Afghanistan, though quantifying it is hard because of a reduced intelligence-gathering capability in the country and “because it’s not like they carry identification cards and register somewhere.”

President Joe Biden has spoken repeatedly of what he calls an “over-the-horizon capability” that he says will enable the U.S. to keep track of terrorism threats from afar.

FBI Director Chris Wray has described that home-grown threat as “metastasizing,” with the number of arrests of white supremacists and racially motivated extremists nearly tripling since his first year on the job.

“My concern is that you can’t compare 2001 to today,” said Bruce Hoffman, a terrorism expert at Georgetown University. There’s a “much vaster and better organized bureaucracy,” he said, but it’s burdened with demands not specifically tied to terrorism.

Hoffman said that although he didn’t think al-Qaida would be able to quickly use Afghanistan as a launchpad for attacks against the U.S., it may re-establish “its coordinating function” in the region to work with and encourage strikes by its affiliates — a patient strategy that may yet be vindicated.

“Terrorist groups don’t conform to train timetables or flight schedules,” Hoffman said. “They do things when it suits them and, as al-Qaida was doing, they quietly lay the foundation in hopes that that foundation will eventually affect or determine their success.”

The concern is resonant enough that Biden administration officials told Congress last week that, based on the evolving situation, they now believe terror groups like al-Qaida may be able to grow much faster than expected.

U.S. authorities alleged last year that a Saudi gunman who killed three U.S. sailors at a military base in Florida in 2019 had communicated with al-Qaida operatives about planning and tactics. Last December, the Justice Department charged a Kenyan man with trying to stage a 9/11-style attack on the U.S. on behalf of the terrorist organization al-Shabab, which is linked to al-Qaida.

Now it’s possible that other extremists will find themselves inspired by al-Qaida, even if not directed by it.

“Until recently, I would have said that the threat from al-Qaida core is pretty modest. They didn’t have safe haven in Afghanistan, their senior leadership was scattered,” said Nathan Sales, former coordinator for counterterrorism at the State Department.

But, now with the Taliban back in control, “all of that could change and could change very rapidly.”