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BOSTON (Reuters) - Computer security experts have discovered a group of highly sophisticated computer hackers operating for hire, a U.S. computer security firm said on Tuesday, and it linked the group to some of the best-known cyber-espionage attacks out of China in recent years.

Symantec Corp said the hacker group, which it dubbed "Hidden Lynx," was among the most technically advanced of several dozen groups believed to be running cyber espionage operations out of China. Unlike a previous report by another company, Symantec did not allege Chinese government involvement in the cyberattacks.

Symantec's 28-page report said its researchers believe the Hidden Lynx group may have been involved with the 2009 Operation Aurora attacks, the most well-known cyber espionage campaign uncovered to date against U.S. companies.

In Operation Aurora, hackers attacked Google Inc and dozens of other companies including Adobe Systems Inc. Google disclosed the attacks in January 2010, in which hackers tried to read Gmail communications of human rights activists and also attempted to access and change source code at targeted companies.

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Symantec researcher Liam O'Murchu said his firm was unable to determine which individuals were behind Hidden Lynx or if it was linked to the Chinese government.

A separate study, released in February from the U.S. computer security firm Mandiant, said a secretive unit of the Chinese military was engaged in cyber espionage on American companies. Beijing vehemently denied the accusations in that document, which contained photos of the building that Mandiant alleged was the unit's headquarters.

Symantec believes the group is based in China, O'Murchu said, because much of the infrastructure used to run the attacks is based there and because the malicious software was written using Chinese tools and with Chinese code.

The Symantec report also provides new details about who is behind several recent attacks, including a breach at cyber security firm Bit9 and follow-on attacks at three Bit9 clients.

It also connects Hidden Lynx to a major campaign dubbed Voho, which was discovered last year by the security firm RSA, which is owned by EMC Corp. Voho targeted hundreds of organizations including financial firms, technology and healthcare companies, defense contractors and government agencies.

Symantec described the Hidden Lynx group as a "professional organization" staffed by between 50 and 100 people with a variety of skills needed to breach networks and exfiltrate data. The arsenal of tools included Trojan Naid and Trojan Moudoor, which the gang use to siphon data from infected computers.

Symantec, which sells software and services to protect corporate and consumer computer systems from cyber attacks like the ones mentioned in the report, said Naid was also used by hackers in Operation Aurora.

The Hidden Lynx hackers "were either responsible for the Aurora attack or were working in conjunction with the Aurora attackers," O'Murchu said.

(Reporting by Jim Finkle; Editing by Richard Valdmanis and David Gregorio)