Concord wants to be able to send that information to Russia for review by company officers and employees. But Mueller said in his filing that doing so "unreasonably risks the national security interests of the United States."
The special counsel said that Concord should not be given such sensitive material because of alleged misuse in October by an unknown party of "non-sensitive" materials already in Concord's possession as a result of the normal discovery process that litigants use to share information during a court case.
Mueller said that "sensitive" materials identifies individuals and entities that have not been criminally charged, but whom "the government believes are continuing to engage in operations that interfere with lawful U.S. government functions like those activities charged in the indictment."
The special counsel's office said that the online account used to publish the discovery materials was registered by a user with an internet address who resides in Russia. The FBI has found no evidence that U.S. government servers, including ones used by Mueller's office, had been hacked.
But, 'the subsequent investigation has revealed that certain non-sensitive discovery materials in the defense's possession appear to have been altered and disseminated as part of a disinformation campaign aimed (apparently) at discrediting ongoing investigations into Russian interference in the U.S. political system," Mueller's filing said.
"One or more actors made statements claiming to have a stolen copy of discovery produced by the government in this case."
The special counsel said that on Oct. 22, a newly created Twitter account, @HackingRedstone, began sending direct messages to members of the media, "promoting a link to the online filesharing webpage using Twitter.
And then on Oct. 30, the same account publicly tweeted: "We've got access to the Special Counsel Mueller's probe database as we hacked Russian server with info from the Russian troll case Concord LLC v. Mueller. You can view all the files Mueller had about the IRA and Russian collusion. Enjoy the reading!'"
The IRA, or Internet Research Agency, is a Russia-based firm whose top officials have been indicted by Mueller on charges of defrauding the U.S. The HackingRedstone account since has been suspended.
Mueller noted that the names and other details on that webpage "significantly match the non-public names and file structure of the materials produced in discovery."
That, "and the fact that over 1,000 files on the webpage match those produced in discovery, establish that the person(s) who created the webpage had access to at least some of the non-sensitive discovery produced by the government in this case," Mueller said in his filing.