Earlier this year, presidential campaign computers in the camps of both Barack Obama and John McCain came under cyberattack from unknown hackers, it was reported this week.

The Obama attack, Newsweek magazine in the US reported, was initially thought to be a piece of malware downloaded from a phishing site. The magazine reported that "the next day, both the FBI and the Secret Service came to the campaign with an ominous warning: 'You have a problem way bigger than what you understand,' an agent told them. 'You have been compromised, and a serious amount of files have been loaded off your system."

The McCain campaign's computer system was also compromised. Newsweek confirmed with a top McCain official that the FBI had become involved. A federal investigation into both attacks is under way.

According to Newsweek editor at large Evan Thomas, the FBI and White House officials told the Obama campaign that a foreign entity or organisation was likely responsible, not political opponents.

Independently, Obama technical experts have speculated that the hackers were Russian or Chinese. The files accessed appear to be policy-related and thus potentially useful in future negotiations with a new presidential administration.

Earlier this year, during the primaries, an online prank had the Obama campaign site redirected to Hillary Clinton's campaign site.