Russian hackers penetrated White House's unclassified computer network in 2014 and accessed President Barack Obama's correspondence, according to a news report.
The hack took place in October at the height of tensions with Russia, which had then annexed Crimea, sent troops into Ukraine and had generally grown more aggressive.
The New York Times said the hackers, who were Russian and may have been a part of the government or working for it, did not, however, gain access to Obama's classified mails.
But there is plenty of sensitive information moving around in the unclassified network - pertaining to scheduling of meetings, movements and debate over policy issues. Many senior US government officials work on two computers, the NYT said. One on the secure, classified network and the other is used for unclassified communications.
The hackers did not penetrate Obama's email account, but managed to get access to some mails sent and received by him by getting into network, through others.
US computer networks are constantly under threat - with the most serious threats coming from Chinese and Russian (and North Korean, as happened last year).
Chinese hackers are known to go after business and commercials entities, while Russians favor political targets, and are known to cover their tracks well.
The October attacks were taken so seriously that the White House email system was partially shut down and officials met daily to take stock for weeks.
Obama is a frequent user of emails - and publicly feuded with the Secret Service to continue using his Blackberry - but is careful when emailing outside the White House system.