Last month, when Google engineers at their sprawling campus in Silicon Valley began to suspect that Chinese intruders were breaking into private Gmail accounts, the company began a secret counteroffensive.

It managed to gain access to a computer in Taiwan that it suspected of being the source of the attacks. Peering inside that machine, company engineers actually saw evidence of the aftermath of the attacks, not only at Google, but also at at least 33 other companies, including Adobe Systems, Northrop Grumman and Juniper Networks, according to a government consultant who has spoken with the investigators.

Seeing the breadth of the problem, they alerted American intelligence and law enforcement officials and worked with them to assemble powerful evidence that the masterminds of the attacks were not in Taiwan, but on the Chinese mainland.

But while much of the evidence, including the sophistication of the attacks, strongly suggested an operation run by Chinese government agencies, or at least approved by them, company engineers could not definitively prove their case. Today that uncertainty, along with concerns about confronting the Chinese without strong evidence, has frozen the Obama administration’s response to the intrusion, one of the biggest cyberattacks of its kind, and to some extent the response of other targets, including some of the most prominent American companies.

President Obama, who has repeatedly warned of the country’s vulnerability to devastating cyberattacks, has said nothing in public about one of the biggest examples since he took office. And the White House, while repeating Mr. Obama’s calls for Internet freedom, has not publicly demanded a Chinese government investigation. Secretary of State Hillary Rodham Clinton, who had been the most senior US official to talk of the seriousness of the breach, discussed it on Thursday with a Chinese diplomat in Washington, however, and a senior administration official said there would be a “démarche in coming days” — a diplomatic move.

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