Department of Homeland Security officials are aware of more than 400 illegal immigrants who were smuggled into the United States at the southern border by an ISIS-affiliated network — most of whom remain at large in the country, according to a new report.

Federal law enforcement is internally tracking and arresting migrants who were released into the U.S. after being encountered at the U.S.-Mexico border. To date, 150 of the 400 have been found and arrested in communities nationwide, but the whereabouts of 50 others are unknown, according to NBC News.