Special counsel Robert Mueller in court filings Tuesday raised concerns that a Russian oligarch known as “Putin’s chef” could get access — via the discovery process in the criminal case against his firm — to sensitive materials, including “information that could thwart the ongoing efforts of the United States to “prevent his continuing criminal activity in Russia and elsewhere outside the United States.”

The filing came in the case that Mueller has brought against Russian individuals and entities accused of facilitating Russia’s election meddling on social media, and in particular, in a back-and-forth between Mueller and the only co-defendant in the indictment who has shown up in court to face the charges.

A point of contention between lawyers for the co-defendant, a firm called Concord Management, and Mueller’s team is whether the firm’s general director Yevgeny Prigozhin should have access to discovery materials that prosecutors say are sensitive and have national security implications.

Prigozhin, a restauranteur with Kremlin ties, is also charged in Mueller’s indictment but has not submitted an appearance in court. His firm, Concord Management, entered a not guilty plea last month to a charge of conspiracy to defraud the United States.

The latest round of court filings are over the details of a protective order the parties are asking the judge to place on the discovery process.

Concord Management’s attorneys are asking that Prigozhin have access to the sensitive materials, and that the company’s law firm in the case is not forced, as Mueller is requesting, to designate a representative to be present whenever the sensitive materials are viewed by other parties involved in the defense.

Mueller, meanwhile, said that Prigozhin should not access the materials deemed sensitive by the government unless he makes an appearance in court. The special counsel has additionally requested that the disclosure of sensitive materials to other Concord employees go through a vetting process involving both the court and government attorneys separate from the prosecution team.

Concord Management’s move to hire U.S. lawyers to fight the charges was a surprising one to those who thought that the case would never see a courtroom, given the unlikelihood that Russan President Vladimir Putin would extradite those named in the February indictment to the U.S.

Concord Management’s lawyers have been aggressive in the proceedings and are insistent that Prigozhin be allowed access to the discovery that prosecutors turn over even though he hasn’t shown up in court — an idea Mueller opposes vehemently.

“Prigozhin’s involvement in these proceedings through Concord Management appears to be an effort to use his co-defendant as a stalking horse, or proxy, by which Prigozhin intends to serve his own personal interests, including evading prosecution, while still being afforded access to sensitive discovery—information that could thwart the ongoing efforts of the United States to prevent his continuing criminal activity in Russia and elsewhere outside the United States,” Mueller said.

He said that banning Prigozhin’s access to the materials was necessary to keep the materials and their details within the United States — an idea that prosecutors and the defense in theory both agree on, Mueller said.

He referenced previous court filings submitted for the judge to review privately that outline “the ongoing law enforcement and national security interests implicated by the discovery in this case.”

“While the government wishes to make this discovery available to U.S. defense counsel, the risk of exposing this material to the Russian government warrants limiting the distribution and discussion of the sensitive materials to within the United States and within the U.S. offices of the defendant’s counsel of record,” Mueller said.

The filing also includes the firm’s corporate records filed with a Russian tax agency that Mueller says shows Concord Management’s “ability to alter and amend its management structure” that would make it possible to “find a way to handle discovery and the pending proceedings by means that U.S. corporate entities commonly use to handle criminal investigations (such as special litigation committees and the hiring of independent directors).”

The records reveal that earlier this year Prigozhin changed his position from Concord Management’s controlling shareholder to its general director. Prior to that change, Dmitriy Valeryevich Utkin had been listed as Concord Management’s general director.

Mueller additionally questioned whether Concord Management’s U.S. lawyers — — Eric Dubelier and Katherine Seikaly — could find themselves with a conflict of interest in representing the firm and other co-defendants the case. (Mueller’s team perviously revealed that U.S. Treasury filings showed that the attorneys also represent Concord Catering, another Russian firm named in the indictment).

“Defense counsel, therefore, may be ethically prohibited from raising the strongest defenses in favor of their client, Concord Management, if those positions are adverse to Prigozhin, whom they do not even represent,” Mueller said. “Even assuming their interests are aligned today, they might not always be so.”

Read the Mueller filings below: