Immigrant rights attorneys are urging a major bus company to stop letting federal agents on board to conduct immigration sweeps.

The American Civil Liberties Union's affiliates in 10 states sent a letter Wednesday to officials for the Greyhound bus company asking them to deny agents permission to board without a warrant or on the U.S. border.

The lawyers say U.S. Customs and Border Protection has been searching buses more often to check the immigration status of travelers, singling out people based on race or their appearance. Border Patrol has the power to operate immigration checkpoints and conduct other activities within 100 miles (160 kilometers) of a U.S. land or coastal border according to a federal law. But the letter says the statutes cannot override the Fourth Amendment, which protects people and businesses against illegal search and seizures.

The ACLU alleges that border officials have been randomly boarding Greyhound buses to search passengers, with Greyhound's blessing.

One such incident occurred in Indio last month, when a Los Angeles resident boarded a Greyhound bus, then was detained because his "shoes looked suspicious," according to the ACLU. Because his shoes looked like those of "someone who had recently crossed the border," agents questioned the man regarding his immigration status.

The incident was one of many the ACLU of Southern California says it has received out of Riverside County, though the ACLU's letter to Greyhound headquarters also addressed similar reports out of Washington, Vermont, New York, Michigan, Florida and Arizona. The ACLU affiliates of New Hampshire, Maine and Texas also joined the drafting of the letter.

"Greyhound is in the business of transporting its passengers safely from place to place," said Jennie Pasquarella, director of immigrants' right for the ACLU Foundations of California. "It should not be in the business of subjecting its passengers to intimidating interrogations, suspicion-less searches, warrantless arrests and the threat of deportation."

According to the ACLU, the raids are being conducted by immigration agents "with the agreement of Greyhound," and the number of searches are comparable to actions of "police states."

Greyhound issued a statement earlier this year saying the company was required to cooperate with law enforcement agents who demand to board the buses, but the ACLU insists agents can be denied access if they lack probable cause. Allowing immigration agents to detain passengers without probable cause is a violation of the passengers' Fourth Amendment rights against unlawful search and seizure, the ACLU argued.