The operation against the top American business lobbying group involved at least 300 Internet addresses and was discovered and shut down in May 2010, the Journal reported, citing unidentified people familiar with the matter.
The newspaper reported it was not known how much information was seen by the hackers, who may have had access to the network for more than a year before being discovered.
The group behind the breach is suspected by the United States of having ties to the Chinese government, one of the sources told the newspaper. The FBI informed the Chamber of Commerce that servers in China were pilfering its information, the source said.