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.

Since the Hamas attacks in Israel on Oct. 7, the F.B.I. has received “more than 1,800 reports of threats or other types of tips or leads that are somehow related to or have a nexus to the current conflict in Israel and Gaza,” said Lisa Monaco, the deputy attorney general, though many of the cases were resolved without incident.

National security officials are deeply concerned about the pace of the threats.

“Looking back over my career in law enforcement, I’d be hard pressed to think of a time when so many different threats to our public safety and national security were so elevated all at once, but that is the case as I sit here today,” the F.B.I. director, Christopher A. Wray, told Congress this month, just days before the Tajiks were arrested.

An F.B.I. spokeswoman declined to comment.

For years, Republicans and conservative media outlets have described the potential dangers posed by terrorists who might slip into the country at the southwestern border along with tens of thousands of Latin American migrants. Those fears, for the most part, have not been realized.

It is still unclear if the men were, in fact, planning a terrorist attack — whether directed by the Islamic State or inspired by the extremist group. But the resources the F.B.I. devoted to the case underscore how seriously the bureau continues to view the threat as a top priority.

The arrests come at a moment of maximum political attention to border security. The issue has emerged as a major source of contention between President Biden and former President Donald J. Trump, who frequently talks about “migrant crime.”

Still, Representative Jim Himes, Democrat of Connecticut and the ranking member of the House Intelligence Committee, urged that the incident be put in context. He cautioned that the “number of fatal terrorist attacks undertaken by undocumented migrants who crossed our southern border is zero” and that the “number of Americans injured by foreign-born terrorists who entered the country illegally is zero.”

Tajik adherents of the Islamic State — especially within an affiliate known as ISIS-K — have taken increasingly high-profile roles in several recent terrorist attacks. Over the last year alone, Tajiks have been involved in assaults in Russia, Iran and Turkey, as well as foiled plots in Europe.

ISIS-K, or the Islamic State Khorasan Province, was founded in Afghanistan in 2015 by disaffected members of the Pakistani Taliban, who then embraced a more violent version of Islam. The group saw its ranks cut roughly in half, to about 1,500 to 2,000 fighters, by 2021 from a combination of American airstrikes and Afghan commando raids that killed many of its leaders.

The group got a second wind soon after the Taliban toppled the Afghan government that year. During the U.S. military withdrawal from the country in August 2021, ISIS-K carried out a suicide bombing at the international airport in Kabul that killed 13 U.S. troops and as many as 170 civilians.

ISIS-K has since revived some of its global ambitions, with Tajiks constituting more than half of its several thousand soldiers, experts said.

Russia is a frequent target, but ISIS-K has also vowed to attack Americans and the United States.

Most of the details surrounding the F.B.I.’s investigation remain secret, but interviews with several U.S. officials familiar with the case have provided additional insights.

The officials said the men entered the United States through the border in Southern California and Texas beginning in 2023. They are all ethnic Tajiks, but at least one had a Russian passport. Some of the men might have known each other.

They made their way to Los Angeles, Philadelphia and New York, where there are large Central Asian populations. Once the F.B.I. determined that the men might have a connection to the Islamic State or sympathize with the group, the bureau managed to figure out their whereabouts.

That set off a sprawling investigation that was reminiscent of the bureau’s efforts after Sept. 11 to track multiple terrorism suspects in thwarted attacks, such as a plot against the New York subways in 2009. In previous high-priority terrorism investigations, F.B.I. has relied on aerial surveillance and a key warrantless surveillance program known as Section 702 to gather intelligence.

The program authorizes the government to collect the communications of foreigners abroad who have been targeted for intelligence purposes, including when those people are interacting with Americans.

The stakes were extremely high for the FB.I. and Mr. Wray. If any of the Tajiks had slipped away and carried out a terrorist attack, the bureau would have been blamed for not apprehending them earlier and faced more withering Republican criticism. Yet there is always a trade-off. Arrests make it harder to gather information about a possible network.

In the case of the Tajiks, officials said, it is still not known what the men were doing, whether they were being directed by a terrorist group outside the United States or had been inspired to carry out an attack on their own.

Whatever the F.B.I. eventually learned about the men’s movements caused bureau counterterrorism officials to take them off the street and have them arrested on immigration charges. Agents with the U.S. Immigration and Customs Enforcement and the F.B.I. picked up the men, who have not been named, over the weekend of June 8 in New York, Los Angeles and Philadelphia.

Federal authorities have not disclosed publicly what led investigators to believe the men might be involved in terrorism. At the time, law enforcement officials said only that the men were arrested after unspecified “derogatory information” about them was discovered.

In a separate case, lawyers representing a group of nationals from Uzbekistan sued the U.S. government in federal court in February, claiming that migrants from that Central Asian country had been targeted for detention at the southern border.

If the Tajiks are held only on immigration charges and not other federal crimes they will almost certainly be deported, officials said.

In his testimony to Congress before the arrests, Mr. Wray hinted at the threat even as the F.B.I. quietly watched the suspects.

“But, now, increasingly concerning is the potential for a coordinated attack here in the homeland, akin to the ISIS-K attack we saw at the Russia Concert Hall in March,” Mr. Wray said.

More than 130 people were killed in that attack near Moscow, and several of the suspects who have been arrested are Tajik.

The post The Southern Border, Terrorism Fears and the Arrests of 8 Tajik Men appeared first on New York Times.