A database of images collected by US Customs and Border Protection has been compromised.

The database included photos of travelers and license plates.

Hackers got access to the collection by compromising the network of one of the agency's subcontractors, CBP said.

The agency declined to say how many photos were affected or when the incident happened.

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Hackers broke into a database of images of travelers and license plates collected by US Customs and Border Protection, the agency said on Monday.

The hackers gained access to the images through a subcontractor's network, CBP said. The subcontractor, which the agency declined to name, had transferred the photographs to its network in violation of CBP policies, the agency said.

"CBP has alerted members of Congress and is working closely with other law enforcement agencies and cybersecurity entities and its own office of professional responsibility to actively investigate the incident," the agency said in a statement.

The agency became aware on May 31 that the subcontractor had transferred the images to its network. CBP did not say when the subcontractor transferred the photographs, when the cyber attack occurred, or how many images were accessed by the hackers.

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Agency spokesman Mike Niezgoda declined to comment on the incident beyond CBP's statement, which he pasted into an email message.

The agency released a separate copy of the statement as a Microsoft Word document that was entitled, "CBP Perceptics Public Statement," according to the Washington Post, which first reported the security breach. The title seemed to indicate that Perceptics, a company that offers license-plate reader technology, was involved in the incident.

Niezgoda declined to confirm whether Perceptics was connected to the breach. Company representatives did not respond to an email seeking comment.

The CBP had been collecting images of travelers at airports and at land border crossings. It has also begun to use facial recognition to identify travelers, including those trying to enter the country illegally.

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