NPR: Federal Agents Board Buses 100 Miles From Border To Ask, Are You A US Citizen? by Samantha Raphelson:
When people are crossing a U.S. border, they expect to be asked about their citizenship. But not when they’re driving up the East Coast.
U.S. Border Patrol agents are boarding buses from private lines like Greyhound and Concord Coach within 100 miles of a U.S. border, asking passengers if they’re American citizens. It turns out agents are empowered to do this through a little-known law called the Immigration and Nationality Act of 1952. There are more and more reports of officers stopping cars and buses.