The claims were made in a filing on Wednesday in his investigation into Russian election interference and alleged collusion with the Trump campaign.
And Reuters reported that a journalist had received a message from a hacker claiming to have hacked a Russian legal firm that had received the evidence from Concord's U.S. law firm, Reed Smith LLP.
Evidence gathered by Robert Mueller, the special counsel, was obtained by Russians and leaked online in an attempt to discredit his inquiry into Moscow's interference in U.S. politics, prosecutors said on Wednesday.
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".
The leak I mentioned in Mueller's documents file to the court with prosecution of Concord Management and Consulting, a Russian company, which was allegedly funding hackers operation by Russia's notorious Internet Research Agency (IRA). Lead Concord attorney Eric A. Dubelier, of Reed Smith in Washington, declined to comment on the Mueller filing, as did Mueller spokesman Peter Carr.
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Independent disinformation researcher Josh Russell said he was also sent the documents from the @HackingRedstone account, telling NBC that he was "super excited to click the link they posted but then it was just a big letdown". Unique identifiers, or "hashtag values", for more than 1,000 files were found to match those of evidentiary files, which were mixed in randomly with "junk material", they said.
The FBI has found no evidence that U.S. servers were compromised, and the IP address of the account used to publish the materials originated in Russian Federation, prosecutors said.
In making the case against further sharing of materials, the government revealed that documents shared by prosecutors and traceable to Mueller's office ended up getting released online - and altered. The filing argued that the sensitive information shouldn't be shared outside the USA because some of the discovery "identifies uncharged individuals and entities that the government believes are continuing to engage in operations that interfere with lawful US government functions like those activities charged in the indictment".
Wednesday's filing comes as the Justice Department, including federal prosecutors in Washington and Mueller's office, fight Concord's attempt to share information in the case with the company's contacts in Russian Federation.
In an indictment a year ago, Mueller's office said Russian defendants adopted false online personas to push divisive messages, travelled to the United States to collect intelligence and orchestrated political rallies while posing as Americans.