The Department of Homeland Security has identified over 400 immigrants who have come to the U.S. from Central Asia and elsewhere as “subjects of concern” because they were brought by an ISIS-affiliated human smuggling network, three U.S. officials tell NBC News.

While over 150 of them have been arrested, the whereabouts of over 50 remain unknown, the officials said, and Immigration and Customs Enforcement is looking to arrest them on immigration charges when they are located, NBC reports.

“In this case, it was the information that suggested a potential tie to ISIS because of some of the individuals involved in [smuggling migrants to the border] that led us to want to take extra care,” said a senior Biden administration official, “and out of an abundance of caution make sure that we exercised our authority in the most expansive and appropriate way to mitigate risk because of this potential connection being made.” 

The official added that since ICE began arresting migrants brought to the U.S. by the ISIS-linked smuggling group several months ago, no information has emerged tying them to a threat to the U.S. homeland. 

Recent terrorist attacks in Russia have fueled heightened concern about ISIS and its offshoot ISIS-K. In recent months, DHS has been looking more closely at migrants from Tajikistan, Uzbekistan, Moldova, Kyrgyzstan, Georgia and Russia, countries where ISIS-K has been active.