(CNN) — The Department of Homeland Security has identified more than 400 migrants who came to the US via a human smuggling network that may have some connection with ISIS, prompting an additional review of those individuals, according to two US officials.

The purpose of the network was to smuggle people, not bring in terrorists, one of the officials said. But it is the same network that brought eight Tajik nationals, who were arrested earlier this month after officials discovered that they had some specific and troubling connections to the terror group.

There was specific “derogatory information” linking those individuals to ISIS, the official said. In the latest case, the 400 migrants under scrutiny – mostly from Central Asian nations – are being screened purely because of their connection to the human smuggling network.

Homeland Security Secretary Alejandro Mayorkas told reporters Wednesday that the department hasn’t identified “400 people with potential ISIS ties.”

Of those 400 people, a number of them have been detained by immigration authorities, according to the official. But there haven’t been cases identified of anyone threatening the US at this point, the official said. Vetting is ongoing.

NBC News was first to report the figure of 400 migrants.

The review comes as US officials have grown increasingly concerned about migrants from Central Asian nations such as Tajikistan and Uzbekistan. The leadership of ISIS-K is mainly made up of Tajik nationals and the group has recruited from both countries. Tajik nationals 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.

US officials have been paying particular attention since last summer, when a group of Uzbek nationals who had crossed the southern border were later found to have been assisted in traveling to the United States by a facilitator who had ties to ISIS.

The episode sparked a scramble across the US government to locate and investigate those people.

Two US officials also said that it spurred national security officials to ensure that immigration and intelligence authorities were appropriately monitoring anyone traveling from Tajikistan, Uzbekistan and Kazakhstan.

“I think what (the incident with the Uzbek nationals) did last summer was suggest Central Asians are potentially a population of concern, given what we know about the global ISIS network right now,” a senior US official previously told CNN.

The more recent episode played out in a similar fashion: The eight men from Tajikistan initially entered the US at the southern border and requested asylum under US immigration law. By the time intelligence that was 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 that 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.

National security officials feared that at least some of the eight Tajiks were ripe for radicalization by ISIS-K while they were inside the United States, potentially struggling with isolation, financial stress or discrimination – all things that could make a person susceptible to ISIS propaganda glorifying violence.

Senior officials now see a so-called lone-wolf attacker who emerges seemingly from nowhere as perhaps the more likely – and potentially equally dangerous – threat rather than the more traditional coordinated plot carried out by trained operatives.

Earlier Wednesday, Mayorkas told MSNBC that “we have no evidence that (the 400) are individuals plotting to harm the United States.”

“We screen and vet individuals at the time of encounter. If we learn of derogatory information, we take enforcement action, we are proceeding with extreme caution here in the service of the security and safety of the American people, number one,” he said.

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