Extremists around the world may be emboldened after seeing the Taliban’s victory in Afghanistan and the U.S. withdrawal, said the head of Britain’s MI5 on Friday. The spy chief’s comments came on the eve of the 20th anniversary of the Sept. 11 attacks that led to the U.S. war in Afghanistan.

Director General Sir Ken McCallum, the head of Britain’s domestic intelligence service, told BBC Radio in an interview that “there was no doubt” that extremists worldwide may be inspired by events in Afghanistan. It is the risk of terrorist acts by inspired extremists that he says MI5 is keenly focused on in its counterterrorism mission today.

The other concern that MI5 and other intelligence agencies have across the world is the risk that Afghanistan under the Taliban will provide safe haven for terrorist groups to plot new attacks. In their final agreement with the U.S., the Taliban was urged to cut all ties with Al-Qaeda, the group responsible for the 9/11 attacks and an ally during the war. The Taliban has pledged that no group would be allowed to threaten other countries from Afghanistan, but a United Nations report from June warned it would be impossible to assess with confidence if it will follow through on suppressing terrorist activities.

"The big concern flowing from Afghanistan alongside the immediate inspirational effect is the risk that terrorists reconstitute and once again pose us more in the way of well-developed, sophisticated plots of the sort that we faced in 9/11 and the years thereafter," McCallum said.

The U.K. was an early ally of the U.S. in the global war on terror and has been the victim of terrorist attacks. On July 7, 2005, a series of bombings in London left 52 dead and 700 injured. In 2017, devotees of the Islamic State (IS) conducted several attacks in the country, including a bombing in Manchester outside an Ariana Grande concert that left 23 dead.

It is this experience with attacks by radicalized lone wolves that concerns McCallum the most, especially after the withering criticism endured by MI5 after failing to prevent them. He noted that it would take time for Afghanistan to potentially become a haven for terrorist planners, but that timeline was much shorter for the self-radicalized or inspired devotees of terror groups.

However, the spy chief conceded that it was likely that not every attack can be prevented and there was a possibility some could even take place under his watch. McCallum said MI5 had "saved thousands of lives across the last 20 years" but acknowledged that it "cannot always succeed."