A cyberattack on a firm that performs background checks for U.S. government employees has compromised data on at least 25,000 workers, including some undercover investigators, and that number could rise, officials said on Friday.

The breach at U.S. Investigations Services (USIS), based in Falls Church, Virginia, exposed highly personal information of workers at the Department of Homeland Security's headquarters as well as its U.S. Immigration and Customs Enforcement and U.S. Customs and Border Protection units, two officials familiar with the investigation into the breach said.

Some employees have already received letters warning them about the breach. The letters say the compromised information includes Social Security numbers, education and criminal history, birth dates, and information about spouses, other relatives and friends, including their names and addresses.