Photos of travelers and their vehicle plates snapped at a U.S. border control point have been hacked, according to the Customs and Border Protection agency.

Customs officials said in a statement on Monday that the hack involves fewer than 100,000 people photographed inside vehicles — as well as images of the vehicle license plates — that were taken as travelers left the U.S. through specific lanes at a single, unspecified land-border crossing. The images were captured by CBP over a six-week period.

The images found their way into the database of a government subcontractor that hackers were able to penetrate, the agency said.

"Initial information indicates that the subcontractor violated mandatory security and privacy protocols outlined in their contract," CBP said in a statement.

Although the agency did not name the subcontractor, the news follows a report last month in the computer security website The Register that Tennessee-based — which purports to be "the sole provider" of license-plate readers at land border points in the U.S. — had been hacked.

CBP, which says it learned of the breach on May 31, says none of the images had yet surfaced on the Internet or Dark Web.

The American Civil Liberties Union, among others, has expressed concern over the lack of regulation of license-plate-reading cameras, that the information from the readers "is being collected and sometimes pooled into regional sharing systems."

"As a result, enormous databases of innocent motorists' location information are growing rapidly," the civil liberties group wrote. "This information is often retained for years or even indefinitely, with few or no restrictions to protect privacy rights."

The Electronic Frontier Foundation, which says it defends civil liberties in the digital world, warns that "Location-based information like license plate data can be very revealing. By matching your car to a particular time, date and location, and then building a database of that information over time, law enforcement can learn where you work and live, what doctor you go to, which religious services you attend, and who your friends are."

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