(CNN) -- USIS, a major contractor providing background checks for the U.S. government, said Wednesday that hackers suspected to be working for a foreign government breached its computer systems.
Government officials said the attackers might have stolen information belonging to U.S. government employees.
The FBI is investigating, and the Department of Homeland Security said it has halted providing employee data to USIS for background checks until it can be assured the data won't be compromised.
The company said in a statement that experts investigating the breach "believe it has all the markings of a state-sponsored attack."
DHS spokesman Peter Boogaard said, "At this time, our forensic analysis has concluded that some DHS personnel may have been affected, and DHS has notified its entire workforce, out of an abundance of caution, to advise them to monitor their financial accounts for suspicious activity."
DHS warned employees to be on guard for unsolicited requests for private and financial information.
USIS has been a beleaguered company of sorts in the past year.
The Justice Department has joined a lawsuit accusing the company of shoddy background check work and defrauding the government. It conducted background checks on NSA leaker Edward Snowden and Aaron Alexis, who carried out the Washington Navy Yard mass shooting.
Government officials have said the Alexis background check met government procedures and that there are no indications the Snowden check was deficient.
USIS said it uncovered the breach recently and reported it to authorities. The company is working with the Office of Personnel Management and the DHS US-Cert cyber-response teams to investigate and mitigate the breach.