The individuals — who sources said have ties to ISIS — crossed into the U.S. via the southwest border between 2023 and 2024 but at the time, immigration officials had no information connecting them to the terrorist group. The eight migrants were arrested by ICE when they entered the U.S. without proper documents and were subsequently released into the U.S. with notices to appear in immigration court, according to a senior official at the Department of Homeland Security.

Federal law enforcement acquired intelligence information indicating the men were inside the U.S. and had likely ties to ISIS, according to two of the sources familiar with the investigation. The FBI Joint Terrorism Task Force opened an investigation.

Investigators started monitoring their communications and followed their activities on social media like chat rooms and encrypted sites, according to one of the sources.