The Khorasan Group is attracting hundreds of new followers in Syria, where its recruiting of disaffected Muslims in the U.S. and Europe makes it one of the most dangerous terrorist groups to America and its allies.

“Syria is a very rich recruiting ground for the Khorasan Group,” which targets primarily first- and second-generation European immigrants because “it’s much easier to train them, motivate them, give them a network and send them back to the west,” said Rohan Gunaratna, head of the International Centre for Political Violence and Terrorism Research at Nanyang Technological University in Singapore.

Gunaratna estimates that “it cannot be more than a few dozen” fighters who’ve gone through this process, “but they present a significant threat, because it will be difficult for Western governments to know who they are.” The group’s capacity to recruit, train and activate terrorists means there probably will be more, he said.