WASHINGTON (AP) -- A Homeland Security Department official says a recent computer breach at a major government security clearance contracting firm may have affected the internal files of as many as 25,000 of the agency's workers.The official says the estimate of Homeland Security workers affected by the breach at USIS may rise further. The official spoke on condition of anonymity in order to discuss details of an incident that is under active federal criminal investigation. Homeland Security will soon begin notifying employees whose files were compromised and urge them to monitor their financial accounts, the official said.A USIS spokeswoman declined to comment. The company said earlier in a statement on its website that the cyberattack appeared to "have the markings of a state-sponsored attack."The FBI is investigating.By STEPHEN BRAUNAssociated PressCopyright 2014 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.