Independent security researchers said that at least 34 corporations had been targets of the attacks originating in China.

Adobe, a software maker, said it had been the victim of an attack, but said that it did not know if it was linked to the hacking of Google. Some reports suggested that Yahoo had been a victim, but a person with knowledge said that Yahoo did not think that it been subject to the same attack as Google.

The decision by Google to draw a line and threaten to end its business operations in China brought attention to reports of Chinese high-technology espionage stretching back at least a decade. But despite Google’s suggestion that the hacking came from within China, it remained unclear who was responsible. Nevertheless, it presented the Obama administration with a problem of how to respond.

Google’s description of the attacks closely matches a vast surveillance system called Ghostnet that was reported in March by a group of Canadian researchers based at the Munk Center for International Studies at the University of Toronto. They found that an automated espionage system based in China was using targeted e-mail messages to compromise thousands of computers in hundreds of governmental organizations. In each case, after the computers were controlled by the attackers, they were able to scan for documents that were then stolen and transferred to a digital storage facility in China.

The researchers stopped short of directly accusing the Chinese government of masterminding the attacks. However, for years there have been reports of attacks planned by so-called patriotic hackers in China, and many American security specialists argue that these are simply irregular elements of the People’s Liberation Army. At the same time, hackers frequently use so-called false flag espionage or denial of service attacks to route their activities through the computers of a third country and hide their identity.

Image A worker in the lobby of Google's office Wednesday in Beijing. Credit... Gilles Sabrie for The New York Times

One of the Canadian researchers said that fellow computer security researchers suspected that the attack on Google and other recent intrusions relied on hackers sending booby-trapped documents that were stored in Adobe’s Acrobat Reader format, which then infect victims’ computers. This method was seen in a recent wave of attacks on the Dalai Lama’s computers. “We’ve seen a huge upsurge in attacks using Adobe Acrobat,” said Greg Walton, an editor at Information Warfare Monitor, a publication of the Canadian research group.