(CNN) US Customs and Border Protection announced Monday photos of travelers and license plates were recently compromised in a data breach.

In a statement, CBP said it learned on May 31 that a subcontractor "had transferred copies of license plate images and traveler images collected by CBP to the subcontractor's company network. The subcontractor's network was subsequently compromised by a malicious cyber-attack."

The agency has notified Congress and is working with law enforcement and cybersecurity entities to "determine the extent of the breach and the appropriate response," according to the statement.

The images involve fewer than 100,000 people, according to initial reports, a CBP spokesperson said. The photographs were taken of travelers in vehicles entering and exiting the United States through a few specific lanes at a single land border port over one and a half months, said CBP.

"No other identifying information was included with the images," said the spokesperson.

Read More