The Journal said the hacking apparently was for monitoring its coverage of China.
The New York Times reported Chinese hackers had been attacking its computer systems for four months until it was stopped by its security experts. The report said the attacks began in September as it was getting ready to publish an article that billions of dollars of personal fortunes had been accumulated by the family of Chinese Premier Wen Jiabao through business dealings.
The Journal, citing those familiar with response to cyberattacks, said such infiltration of its networks along with those of other news outlets "indicate that Chinese spying on U.S. media has become a widespread phenomenon." The report said Chinese hackers for years have targeted major U.S. media companies.
The Journal, quoting its sources, said the U.S. Federal Bureau of Investigation has been probing such incidents for more than a year as the hacking is seen as a national-security issue against U.S. interests.
"Evidence shows that infiltration efforts target the monitoring of the Journal's coverage of China and are not an attempt to gain commercial advantage or to misappropriate customer information," said a statement from Paula Keve, spokeswoman for Journal's parent Dow Jones & Co., which is a unit of News Corp.
Noting the infiltration is an "ongoing issue," she said: "We continue to work closely with the authorities and outside security specialists, taking extensive measures to protect our customers, employees, journalists and sources."
Keve was quoted as saying the Journal on Thursday completed a network overhaul to bolster security, adding her organization intends to continue "the aggressive and independent journalism for which we are known."
The Journal, quoting its sources, said penetrating its computer system through its Beijing bureau was one of the ways adopted by the hackers.
The New York Times said it asked its computer network monitor AT&T to look for unusual activity after Chinese officials reportedly talked of consequences for the newspaper's article on the Wen family. The attacks on its systems included infiltrating its computer systems and getting passwords for its reporters and other employees.
Since its computer security experts helped build better defenses to block the infiltration, the hackers haven't been able to break back into its systems, The Time said.
"Security experts hired by the Times to detect and block the computer attacks gathered digital evidence that Chinese hackers, using methods that some consultants have associated with the Chinese military in the past, breached the Times's network," the report said.
Times Executive Editor Jill Abramson said experts found no evidence that sensitive email or files from the reporting on Wen's family were accessed, downloaded or copied. The Times also quoted its experts that no customer data were stolen from the newspaper.
The Times quoted China's Ministry of National Defense that its laws prohibit any hacking that damages Internet security and that "to accuse the Chinese military of launching cyberattacks without solid proof is unprofessional and baseless."