The Southern Border, Terrorism Fears and the Arrests of 8 Tajik Men

When eight Tajik men sought asylum at the southwestern U.S. border months ago, federal authorities had no reason to doubt that they were desperate migrants fleeing a poor country in war-torn Central Asia.

But soon after they were admitted into the country, the F.B.I. learned they might have ties to the Islamic State and opened a counterterrorism investigation.

This was no ordinary inquiry. Dozens of personnel monitored the men closely as they made their way to different cities across the United States, officials said. The White House was updated regularly.

The bureau hoped to gather information about a broader terrorist network. But heightened concerns about an imminent attack in at least one location triggered the arrest of all eight men earlier this month on immigration charges, according to several U.S. officials speaking on the condition of anonymity to discuss the sensitive investigation. So far, the men have not been charged with any terrorism-related offenses.

The dramatic episode unfolded as anxiety has risen among U.S. officials, who have been warning for months that the conflict in Gaza and unrest in Central Asia could spill into the United States, most likely in the form of small radicalized groups acting on their own initiative or lone-wolf terrorists.

The new details about the F.B.I. investigation and the decision to arrest the men underscore the deluge of terrorism threats now inundating national security agencies, some emanating from well-known international actors, others from emerging hot spots like Tajikistan.