The U.S. Border Patrol is monitoring millions of American drivers nationwide in a secretive program to identify and detain people whose travel patterns it deems suspicious.

Once limited to policing the nation’s boundaries, the Border Patrol has built a surveillance system stretching into the country’s interior that can monitor ordinary Americans’ daily actions and connections for anomalies instead of simply targeting wanted suspects. Started about a decade ago to fight illegal border-related activities and the trafficking of both drugs and people, it has expanded over the past five years.