USIS, an US government contractor has said that there was a cyber attack on it which had the markings of a state-sponsored attack. An official with the Department of Homeland Security said the intrusion may have compromised some of its employees’ information. The contractor oversees candidates for security clearance for civilian and military works.

USIS, once known as U.S. Investigations Services, Inc., has been under fire in Congress in recent months for its performance in conducting background checks on National Security Agency leaker Edward Snowden and on Aaron Alexis, a military contractor employee who killed 12 people during shootings at the Navy Yard in September 2013.

Private contractors conduct background checks on more than two-thirds of the 4.9 million government workers with security clearances and USIS handles nearly half of that number. Many of those investigations are performed under contracts with the Office of Personnel Management, the Department of Homeland Security and the Defense Department.