A leading civil liberties group is suing Homeland Security, claiming the agency is keeping the details of its airport face recognition program secret, which it says raises “profound civil liberties concerns.”

The American Civil Liberties Union filed the lawsuit in a New York federal court on Thursday, demanding that the agency turn over records to understand the scope of its airport face recognition system. The group wants to know who Homeland Security works with — including private companies and airlines — as well as internal policies and guidance on how the system is used.

The face recognition system is part of a recent government initiative to scan the faces of travelers both arriving and departing the United States. Homeland Security claims the system will help crack down on immigration violations, such as visitors overstaying their visas.

Although U.S. citizens can opt-out of having their faces scanned, it’s not always openly advertised.

More than a dozen U.S. airports have rolled out the face scanning technology, with many more to go before the U.S. government hits its target of enrolling the largest 20 airports in the country by 2021.

Yet, the lawsuit says there has been “little information” related to the face-scanning system, how it works, the nature of the airline partnerships, or about the privacy safeguards governing the processing and retention of the collected face data. The ACLU said that its recent Freedom of Information Act request to better understand the system has been ignored. Now it’s suing to ask a judge to turn over the documents.

A Homeland Security spokesperson did not immediately comment.

The lawsuit comes just months after the ACLU challenged an effort by Homeland Security to make the face recognition system mandatory for U.S. citizens. Homeland Security backed down from it plans shortly thereafter.