The American Civil Liberties Union (ACLU) brought a lawsuit on Thursday against US Customs and Border Protection (CBP) on behalf of two women who were detained in Montana after a Border Patrol officer heard them speaking Spanish.

Ana Suda and Martha Hernandez, both US citizens, were in line to buy groceries at a convenience store when they were approached by a Border Patrol officer who asked where they were born and requested to see their driver’s licenses. After they handed him their identification, the officer detained the women for 30-40 minutes in the parking lot until back-up arrived, at which point they were released.

The ACLU’s complaint, brought in the US District Court for the District of Montana, alleges that the arrest was unconstitutional on two grounds. First, the ACLU argues that speaking Spanish does not constitute “probable cause” to justify an arrest and thus the officer’s conduct violated Fourth Amendment search and seizure provisions. Second, the complaint alleges that the arrest violated Suda and Hernandez’s right to equal protection under the Fourteenth Amendment because the officer “singled them out based on race, relying on their use of Spanish as a justification and proxy for race.”

The Immigration and Nationality Act of 1946 grants broad discretion to border patrol agents operating “within a reasonable distance from the border.” Havre, Montana, where the women were detained, is located 35 miles from the Canadian border.

CBP policy prohibits its officers from considering race or ethnicity when decided whom to interrogate or detain, but it does not currently restrict its agents from considering language. The ACLU’s complaint requests an injunction to prevent CBP agents from “stopping and/or detaining individuals on the basis of race, accent, and/or speaking Spanish, except where the seizure is based on a specific and reliable suspect description matching such characteristics.” The complaint also requests compensatory and punitive damages for Suda and Hernandez.