Greyhound Lines Inc., the largest U.S. intercity bus company, has threatened to fire employees who sell bus tickets to illegal immigrants under an internal policy that some Hispanic advocacy groups are calling an invitation to racial profiling.

The “Transportation of Illegal Aliens” policy warns Greyhound’s customer service employees to beware of people in large groups, moving in single file and traveling with little or no luggage. It says other telltale signs include people “trying to hide or stay out of plain view” or large groups led by a “guide” who holds everyone’s tickets.