The U.S. Customs and Border Protection Agency (CPB) announced Tuesday that hackers have stolen photos of travelers and their license plates that were taken at the U.S.-Mexico border. The agency first got news of the breach on May 31. 

Fewer than 100,000 photos of travelers were stolen as part of the hack. The hackers were able to steal the images from the network of a subcontractor that does business with the U.S. government. 

"CBP learned that a subcontractor, in violation of CBP policies and without CBP's authorization or knowledge, had transferred copies of license plate images and traveler images collected by CBP to the subcontractor's company network," the CBP said in a statement. "Initial information indicates that the subcontractor violated mandatory security and privacy protocols outlined in their contract."

The subcontractor is likely to be Perceptics, which claims to be the "sole provider" of license plate readers at the U.S. land borders. 

The American Civil Liberties Union (ACLU) has raised concerns towards license plate readers in the past. In a July 2013 report called "You Are Being Tracked' the organization said the readers "serve as a tool for mass surveillance" and that the "information is often retained for years, or even indefinitely, with few or no restrictions to protect privacy rights."

"This incident should be a lesson those who have supported expanding government surveillance powers – these vast troves of Americans' personal information are a ripe target for hackers," U.S. Sen. Rob Wyden said in regards to the cyberattack. 

George Grachis, a senior security and compliance specialist, said that the United States is falling behind when it comes to cybersecurity. "It's time for our country to stop the partisan politics," he wrote on Tuesday. "This behavior is so wasteful and unproductive. While the endless fighting and division continues, cyber criminals who don't work in silos are all too happy to exploit our banks, medical records, military secrets and intellectual property."