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– Special Counsel Robert Mueller warned that Russian intelligence services still have active “interference operations” into U.S. elections and that handing over certain evidence in a criminal case could imperil ongoing investigations.

Mueller on Tuesday asked a federal judge in Washington for an order to protect voluminous evidence sought by lawyers for Concord Management and Consulting LLC, one of three companies and 13 Russian nationals charged in a February indictment alleging election meddling via social media. Prosecutors have uncovered evidence of other individuals and entities who are “continuing to engage” in similar activities.

The legal battle between Mueller and Concord’s attorneys highlights the tension between intelligence-gathering, which is cloaked in secrecy, and the U.S. legal system, which entitles criminal defendants to review evidence against them. Lawyers for Concord, a company linked to a longtime associate of Russian President Vladimir Putin, have been negotiating to obtain documents from the special counsel to mount a legal defense.

The Russians are accused of producing propaganda, posing as U.S. activists and posting political content on social media as so-called trolls to encourage strife in the U.S. The evidence includes between 1.5 and 2 terabytes of data and involves U.S. residents not charged with crimes who the government says were unwittingly recruited by Russians to engage in political activity, prosecutors wrote.

Unauthorized disclosure of such evidence would help foreign intelligence services in Russia and elsewhere while undermining U.S. law enforcement and national security investigations, Mueller’s prosecutors wrote in Tuesday’s request for a protective order.

U.S. documents identify “sources, methods and techniques used to identify the foreign actors behind these interference operations,” Mueller wrote. Improper disclosure of that information would tip off foreign intelligence services about how the U.S. operates and let them “adjust their conduct, thus undermining ongoing and future national security operations.”

Prosecutors said they have gathered “unclassified but sensitive information that remains relevant to ongoing national security investigations and efforts to protect the integrity of future U.S. elections.” It includes the identities of cooperating individuals and companies, as well as links between the defendants, uncharged parties and foreign governments — going well beyond what prosecutors intend to disclose at trial, they wrote. They said they gave additional details to U.S. District Judge Dabney Friedrich under court seal.

Prosecutors asked Friedrich to bar any co-defendants of Concord from reviewing evidence until they appear in his courtroom to respond to the charges. They also want to regulate disclosure of “particularly sensitive material to foreign nationals” by first limiting it to U.S. lawyers for defendants.

Concord, based in St. Petersburg, Russia, is the only defendant to respond in court to the indictment, pleading not guilty May 9. The firm is controlled by Yevgeny Prigozhin, a Russian caterer who is close to Putin. It provides food services to the Kremlin.

Defense lawyers for Concord say they should be able to share evidence with Prigozhin, but Mueller disagrees. “As long as Prigozhin chooses not to appear personally in front of this court, he is not entitled to review any discovery in this case,” the filing said.