Chinese hackers repeatedly penetrated The New York Times' computer systems over the past four months, the newspaper has reported.
It said hacker stole reporters' passwords and hunted for files on an investigation into the wealth amassed by the family of a top Chinese leader.
Security experts hired to investigate and plug the breach found that the attacks used tactics similar to ones used in previous hacking incidents traced to China, the report said.
It said the hackers routed the attacks through computers at US universities.
They installed a strain of malicious software, or malware, associated with Chinese hackers and initiated the attacks from university computers previously used by the Chinese military to attack US military contractors.
The attacks, which began in mid-September, coincided with a Times investigation into how the relatives and family of Premier Wen Jiabao built a fortune worth over $2bn.
The report, which was posted online 25 October 2012, embarrassed the Communist Party leadership.
It came ahead of a fraught transition to new leaders and exposing deep-seated favouritism at a time when many Chinese are upset about a wealth gap.
Over the months of cyber-incursions, the hackers eventually lifted the computer passwords of all Times employees and used them to get into the personal computers of 53 employees.
The report said none of the Times' customer data was compromised and that information about the investigation into the Wen family remained protected, though it left unclear what data or communications the infiltrators accessed.
"Computer security experts found no evidence that sensitive e-mails or files from the reporting of our articles about the Wen family were accessed, downloaded or copied," the report quoted executive editor Jill Abramson as saying.
A Times' spokeswoman declined to comment further.
A Chinese Foreign Ministry spokesman called the Times' accusations groundless and reiterated the government's position that China also has been hacked repeatedly.
"To rashly jump to conclusions based on investigation results which have not been proved by evidence is totally irresponsible behaviour," spokesman Hong Lei said at a routine daily media briefing.
China has been accused by the US, other foreign governments and computer security experts of mounting a widespread, aggressive cyber-spying campaign for several years, trying to steal classified information and corporate secrets and to intimidate critics.
Foreign reporters and news media, including The Associated Press, have been among the targets of attacks intended to uncover the identities of sources for news stories and to stifle critical reports about the Chinese government.