More than 70 organisations are reported to have been targeted worldwide in what software security company McAfee says is the largest ever bunch of cyber attacks.
McAfee, which uncovered the intrusions, declined to name the one "state actor" it believes is behind the attacks, the BBC reports.
However, a cyber expert with the Center for Strategic and International Studies, Jim Lewis, who was briefed on the hacking, says the evidence points to China.
McAfee says it discovered the extent of the hacking in March this year and was taken aback by the audacity of the perpetrators.
Seventy-two organisations were targeted, including the American, Indian and Taiwanese governments, the International Olympic Committee, major defence companies and hi-tech enterprises.
The hackers are said to have broken into the computer system of the United Nations Secretariat in Geneva in 2008 and quietly combed through reams of secret data for almost two years.