WASHINGTON (Reuters) - U.S. Special Counsel Robert Mueller’s office said on Wednesday that self-proclaimed hackers in Russia stole evidence in an attempt to tarnish its investigation of a firm charged with funding a Russian propaganda campaign to interfere in the 2016 U.S. election.

Prosecutors said in a court filing in Washington that a Twitter handle called @HackingRedstone came online last Oct. 22 to brag it had hacked some of the evidence in the case.

“We’ve got access to the Special Counsel Mueller’s probe database as we hacked Russian server with info from the Russian troll case,” the court document quoted the Twitter post as saying.

In February 2018, Mueller indicted 13 Russians and three Russian companies with allegations of tampering in 2016 to support then-Republican candidate Donald Trump. In all, 34 people have pleaded guilty, been indicted or otherwise swept up in the broader inquiry.

The companies named in the indictment included the Internet Research Agency (IRA), known for its “trolling” on social media, Concord Management and Consulting LLC, which is said to have provided financial backing for the operation, and Concord Catering.

The Twitter account linked to an online file sharing portal which it said contained Mueller’s documents about the “IRA and Russian collusion”

“Enjoy the reading!” it added.

The data that appeared online was “altered and disseminated as part of a disinformation campaign aimed (apparently) at discrediting ongoing investigations into Russian interference in the U.S. political system,” prosecutors wrote.

On the same day, a journalist contacted Mueller’s office to report receiving a Twitter message from someone who said they had hacked a Russian legal firm that had received the evidence from Concord’s U.S. based law firm Reed Smith LLP.

FILE PHOTO: Special Counsel Robert Mueller departs after briefing the U.S. House Intelligence Committee on his investigation of potential collusion between Russia and the Trump campaign on Capitol Hill in Washington, U.S., June 20, 2017. REUTERS/Aaron P. Bernstein/File Photo

The illicit activity outlined by prosecutors illustrates the concerns of U.S. intelligence officials about continuing efforts by Russia to interfere in U.S. politics.

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 Russia, prosecutors said.

Concord Management is being prosecuted in Mueller’s investigation of U.S. allegations that Moscow meddled in the American democratic process to help Trump defeat Democrat Hillary Clinton.

Mueller is also investigating whether there was any coordination between members of the Trump campaign and Moscow officials. The Kremlin denies election interference and President Trump denies there was any collusion, calling the inquiry a political witch hunt.

Wednesday’s filing in the U.S. District Court for the District of Columbia is the latest in a dispute between prosecutors and Concord’s American attorneys over how the defense team may share highly sensitive evidence with Concord’s Russian corporate officers.

One of those officers is businessman Evgeny Prigozhin, an associate of Russian President Vladimir Putin and known in Russia as “Putin’s cook.”

In the indictment, prosecutors said Russian defendants adopted false online personas to push divisive messages, traveled to the United States to collect intelligence and orchestrated political rallies while posing as Americans. Prigozhin was one of the 13 people indicted.

He is not expected to appear in a U.S. court because Russia does not have an extradition treaty with the United States. His company Concord did hire American lawyers, however, to fight the charges.

Prosecutors say they do not oppose allowing Concord’s corporate officers to see the evidence.

However, they fear U.S. intelligence sources and methods could be compromised if the materials are not reviewed in the United States. They have asked the judge not to let Concord distribute the materials electronically to people in Russia.