SAN FRANCISCO -- No direct evidence suggests a particular nation such as China is behind Operation Shady RAT, a five-year cyber campaign discovered by McAfee, a cybersecurity expert of the company told Xinhua on Thursday.

Dmitri Alperovitch, a cybersecurity expert and vice president of threat research for software firm McAfee, unveiled in a report Tuesday night that he and his team had discovered Operation Shady RAT, an unprecedented series of cyber attacks on 72 government agencies and business organizations globally.

Alperovitch told Xinhua that they "don't have direct evidence that conclusively points to a particular nation state" behind the scheme.

The McAfee report on Tuesday had said that the campaign was likely sponsored by a nation state because of the breadth and tenacity of the attacks and the information that was accessed.

The attacks were discovered in March this year when McAfee researchers discovered logs of the attacks on a server it was reviewing as part of an investigation into the 2009 defense company data breaches.

The earliest intrusions found so far date back to mid-2006, possibly even earlier.

After the McAfee report was released, Jim Lewis, a cybersecurity expert with the U.S. Center for Strategic and International Studies, told U.S. media that Russia and China could be behind the attacks.