Russian hackers have made off with unclassified emails sent and received by US President Barack Obama as part of an October Whitehouse breach previously played down as minor, according to reports.
The emails could contain sensitive information including diplomatic correspondence, schedules, and policy debate, unnamed officials tell the New York Times.
The White House says the attackers did not breach classified networks nor Obama's personal Blackberry.
The highly-sensitive Joint Worldwide Intelligence Communications System in which the White House, State Department, Pentagon, and intelligence communities share data is said to have been untouched.
The White House and FBI declined to comment.
Much of the detail about what was accessed remains off the public record, including the number of emails stolen or the severity of compromise. Officials say they do not want to reveal the information which could give the hackers insight into the Government investigation.
One official briefed on the attack described the attackers as "one of the most sophisticated" the US Government had seen, triggering several weeks of daily briefings.
Hackers also accessed the less secure State Department network close to the time of the attack.
White House and State Department sources say the Russia hackers likely had ties if not direct support from Moscow, but attribution has not been officially confirmed.
This could be seen as marking a departure from the naming-and-attempted-shaming exercise by the US Government when it pinned the 2014 hack of Sony Pictures on North Korea, allegations Pyongyang refutes. ®