On the outskirts of Shanghai, in a run-down neighborhood dominated by a 12-story white office tower, sits a People's Liberation Army base for China's growing corps of cyberwarriors.
The building off Datong Road, surrounded by restaurants, massage parlors and a wine importer, is the headquarters of P.L.A. Unit 61398. A growing body of digital forensic evidence - confirmed by American intelligence officials who say they have tapped into the activity of the army unit for years - leaves little doubt that an overwhelming percentage of the attacks on American corporations, organizations and government agencies originate in and around the white tower.
An unusually detailed 60-page study, to be released Tuesday by Mandiant, an American computer security firm, tracks for the first time individual members of the most sophisticated of the Chinese hacking groups - known to many of its victims in the United States as "Comment Crew" or "Shanghai Group" - to the doorstep of the military unit's headquarters. The firm was not able to place the hackers inside the 12-story building, but makes a case there is no other plausible explanation for why so many attacks come out of one comparatively small area.
"Either they are coming from inside Unit 61398," said Kevin Mandia, the founder and chief executive of Mandiant, in an interview last week, "or the people who run the most-controlled, most-monitored Internet networks in the world are clueless about thousands of people generating attacks from this one neighborhood."
Other security firms that have tracked "Comment Crew" say they also believe the group is state-sponsored, and a recent classified National Intelligence Estimate, issued as a consensus document for all 16 of the United States intelligence agencies, makes a strong case that many of these hacking groups are either run by army officers or are contractors working for commands like Unit 61398, according to officials with knowledge of its classified content.
Mandiant provided an advance copy of its report to The New York Times, saying it hoped to "bring visibility to the issues addressed in the report." Times reporters then tested the conclusions with other experts, both inside and outside government, who have examined links between the hacking groups and the army (Mandiant was hired by The New York Times Company to investigate a sophisticated Chinese-origin attack on its news operations, but concluded it was not the work of Comment Crew, but another Chinese group. The firm is not currently working for the Times Company but it is in discussions about a business relationship.)
While Comment Crew has drained terabytes of data from companies like Coca-Cola, increasingly its focus is on companies involved in the critical infrastructure of the United States - its electrical power grid, gas lines and waterworks. According to the security researchers, one target was a company with remote access to more than 60 percent of oil and gas pipelines in North America. The unit was also among those that attacked the computer security firm RSA, whose computer codes protect confidential corporate and government databases.
Contacted Monday, officials at the Chinese embassy in Washington again insisted that their government does not engage in computer hacking, and that such activity is illegal. They describe China itself as a victim of computer hacking, and point out, accurately, that there are many hacking groups inside the United States. But in recent years the Chinese attacks have grown significantly, security researchers say. Mandiant has detected more than 140 Comment Crew intrusions since 2006. American intelligence agencies and private security firms that track many of the 20 or so other Chinese groups every day say those groups appear to be contractors with links to the unit.
And the Chinese Ministry of Foreign Affairs said Tuesday that the allegations were ‘‘unprofessional.''
‘‘Making unfounded accusations based on preliminary results is both irresponsible and unprofessional, and is not helpful for the resolution of the relevant problem,'' said Hong Lei, a ministry spokesman. ‘‘China resolutely opposes hacking actions and has established relevant laws and regulations and taken strict law enforcement measures to defend against online hacking activities.''
While the unit's existence and operations are considered a Chinese state secret, Representative Mike Rogers of Michigan, the Republican chairman of the House Intelligence Committee, said in an interview that the Mandiant report was "completely consistent with the type of activity the Intelligence Committee has been seeing for some time."
The White House said it was "aware" of the Mandiant report, and Tommy Vietor, the spokesman for the National Security Council, said, "We have repeatedly raised our concerns at the highest levels about cybertheft with senior Chinese officials, including in the military, and we will continue to do so."
The United States government is planning to begin a more aggressive defense against Chinese hacking groups, starting on Tuesday. Under a directive signed by President Obama last week, the government plans to share with American Internet providers information it has gathered about the unique digital signatures of the largest of the groups, including Comment Crew and others emanating from near where Unit 61398 is based.
But the government warnings will not explicitly link those groups, or the giant computer servers they use, to the Chinese army. The question of whether to publicly name the unit and accuse it of widespread theft is the subject of ongoing debate.
"There are huge diplomatic sensitivities here," said one intelligence official, with frustration in his voice.
But Obama administration officials say they are planning to tell China's new leaders in coming weeks that the volume and sophistication of the attacks have become so intense that they threaten the fundamental relationship between Washington and Beijing.
The United States government also has cyberwarriors. Working with Israel, the United States has used malicious software called Stuxnet to disrupt Iran's uranium enrichment program. But government officials insist they operate under strict, if classified, rules that bar using offensive weapons for nonmilitary purposes or stealing corporate data.
The United States finds itself in something of an asymmetrical digital war with China. "In the cold war, we were focused every day on the nuclear command centers around Moscow," one senior defense official said recently. "Today, it's fair to say that we worry as much about the computer servers in Shanghai."