Researchers have been closing in on the unidentified criminals responsible for the attack over the past month. In the process, they have uncovered another 68 so-called command-and-control servers, used to control the hacked machines.

Investigators had already identified 34 hacked companies after examining the single command-and-control server used in the Google attack, and the discovery of another 68 servers could mean that many more companies were compromised than previously thought. "It's easily over 100 companies," said Alex Stamos a partner with Isec Partners.

In the weeks since Google went public with details of the hack, informal discussion lists have sprung up, including security experts and staffers from companies that have been compromised. In those discussions, "that list of control machines keeps getting longer and longer," Stamos said.

The code used in the attacks, known in security circles as , has been in use for at least 18 months, Stamos said. But the security industry was unaware of Aurora until Google discovered the intrusion last December. That allowed hackers to get onto corporate networks undetected.

Other technology companies, including Intel, Adobe, and Symantec, have also been hit by the attack, which investigators have traced back to China.