For possibly more than a year, hackers in China managed to breach the computer systems at the U.S. Chamber of Commerce.
The complex operation was quietly shut down in May 2010 and involved at least 300 Internet addresses.
Hackers gained access to everything stored in the department's system, including information on its 3 million members.
According to the Wall Street Journal, the hackers had access to the system for more than a year before the FBI shut them down.
The Chamber shut down the operation by unplugging and destroying some computers and overhauling its security system.
It's unclear how much data the hackers were able to see.
The Chamber represents millions of U.S. businesses -- primarily small businesses with 100 employees or less.