The material had been handed over to defense attorneys for Concord Management and Consulting LLC, a Russian company that Mueller has charged with financing efforts to meddle in the 2016 U.S. presidential election.
But the files, which prosecutors say were not sensitive, surfaced online last year in a link posted by a pro-Russian Twitter account. The prosecutors stopped short of accusing Concord of leaking the material, but they argue that the company's request to have sensitive new evidence sent to Russia "unreasonably risks the national security interests of the United States."
Concord is controlled by Yevgeny Prigozhin, a wealthy businessman known as "Putin's chef" for his ties to Russian President Vladimir Putin. He has been hit with U.S. sanctions over Russian interference in the 2016 election and is charged alongside his company in the indictment brought by Mueller.
The company is one of three entities and 13 individuals charged in a conspiracy to spread disinformation on social media during Donald Trump's successful presidential bid. Concord is the only defendant to respond to the allegations in court, which means its attorneys have access to evidence in the case.
In Wednesday's filing, prosecutors cited a Twitter account that surfaced last year purporting to have a stolen copy of evidence provided to the company. A tweet from the account @HackingRedstone included a link to a webpage that contained file folder names that matched the materials that Mueller had produced.
The tweet read, "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!"
Prosecutors say the vast majority of the files released by the Twitter account were actually "junk material that has nothing to do with the case," and were included along with real files from the investigation in an attempt to discredit it. An FBI analysis of the files found that only about 1,000 of the 300,000 released were real documents provided to Concord by Mueller's team. The Mueller documents were largely "images of political memes from Facebook and other social media" that were posted during the 2016 election by the Internet Research Agency, a Russian social media troll farm. Concord is accused of funding IRA.
Prosecutors say that whoever created the webpage had access to at least some of the material provided by Mueller in the case. They also say the FBI determined the documents were not stolen from government servers.
Eric Dubelier, a lawyer for Concord, did not immediately return a phone message seeking comment.
This story has been corrected to show that the court filing was Wednesday, not Tuesday.