Members of the group initially entered the US at the southern border and requested asylum under US immigration law. It’s unclear whether they entered at the same time and place.

By the time intelligence collected on overseas ISIS targets connected the men to the terror group, they had already been vetted by immigration authorities and allowed into the country, officials said.

Though there is no hard evidence indicating they were sent to the US as part of a terror plot, at least some of the Tajik nationals had expressed extremist rhetoric in their communications, either on social media or in direct private communications that US intelligence was able to monitor, three officials said.

That discovery set off a flurry of emergency investigative efforts by federal agents and analysts across the country, sources said, including physical and electronic surveillance of the men — a counterterrorism operation reminiscent of the years immediately following 9/11, when the FBI investigated numerous homegrown plots.

After a period of surveillance, federal officials in recent days faced a difficult decision: whether to continue surveilling the men in order to determine if they were part of any potential plot or wider terrorist network, or to move in and take them off the street. Rather than risk the worst-case scenario of a potential attack, senior US officials decided to move in and have the men apprehended by ICE agents, one source told CNN.

The men remain in federal custody on immigration charges and will eventually be deported following the counterterror investigation into them.

Tajiks recruited by ISIS

Of particular concern to US officials was that the men hail from Tajikistan, a corner of Central Asia that in recent years has been a source of steady recruitment by ISIS-K, the Afghanistan-based affiliate of the Islamic terrorist group. ISIS-K is led primarily by Tajiks, who have carried out a series of recent attacks in Europe on behalf of the group, including the Crocus Hall attack in Moscow in March that killed more than 100 people.

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