MIAMI — 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.

The advocates say the "warrantless" checks have taken place in at least seven states including California, Florida and Vermont. Last month, advocates in Florida warned immigrants about the checks when traveling to the state.

A Greyhound spokeswoman did not immediately respond to requests for comment.

Read more: Advocacy groups to immigrants: Don't travel to Florida

An incident early this year on a Greyhound Bus made waves in the Florida immigrant community. During a Jan. 19 immigration sweep in Fort Lauderdale, U.S. Customs and Border Protection agents boarded a bus to detain a woman who they said had overstayed a tourist visa.

A video of the immigration check posted to Twitter by the advocacy group Florida Immigrant Coalition quickly went viral.

The sweep drew the ire of several Florida congressional Democrats.

"We were appalled to see U.S. Customs and Border Protection agents abusing their mandate and authority to arbitrarily board a bus to demand that all passengers produce identification and documentation," Reps. Ted Deutch, Alcee Hastings, Frederica Wilson, Debbie Wasserman Schultz, Darren Soto, Al Lawson and Kathy Cator wrote in a joint January statement.

CBP argued at the time that it was well within its rights to perform the inspection.

"Border Patrol agents routinely conduct law enforcement activities at transportation hubs as part of a layered approach to preventing illegal aliens from traveling further into the United States," the agency said in a statement in January.

Times staff writer Kirby Wilson contributed to this report.