Washington (CNN)—Russian hackers behind the damaging cyber intrusion of the State Department in recent months used that perch to penetrate sensitive parts of the White House computer system, according to U.S. officials briefed on the investigation.

While the White House has said the breach only affected an unclassified system, that description belies the seriousness of the intrusion. The hackers had access to sensitive information such as real-time non-public details of the president's schedule. While such information is not classified, it is still highly sensitive and prized by foreign intelligence agencies, U.S. officials say.

The White House in October said it noticed suspicious activity in the unclassified network that serves the executive office of the president. The system has been shut down periodically to allow for security upgrades.