Prosecutors working for the special counsel Robert Mueller indicated Thursday that their case against several Russian entities accused of carrying out a social-media disinformation scheme to interfere in the 2016 election is tied to an ongoing "matter occurring before the grand jury."

The revelation prompted fervent observers of the Russia probe to say that prosecutors may be referring to a separate and secret legal battle between Mueller and an unknown foreign company fighting a grand-jury subpoena.

The subpoena case has been shrouded in mystery since last summer, and the Supreme Court is weighing whether to force the unidentified company to pay fines that were imposed on it by a lower court for refusing to comply with Mueller's subpoena.

In a new court filing Thursday, the special counsel Robert Mueller's team asked the US District Court in Washington, DC, to file a sealed response to lawyers representing an indicted Russian company because the government's response "discusses 'a matter occurring before the grand jury.'"

Fervent observers of Mueller's investigation into Russia's election interference were quick to say that the court filing could be tied to a separate and ongoing legal battle between Mueller and an unknown entity over a grand-jury subpoena.

Thursday's filing was related to Mueller's case against Concord Management and Consulting LLC and the Internet Research Agency (IRA), two of three Russian entities the special counsel indicted last year on charges of conspiring to interfere in the 2016 US presidential election via a social-media disinformation campaign.

Concord is owned by Yevgeny Prigozhin, a Kremlin-allied Russian oligarch who was also indicted and is accused of using his companies to bankroll the IRA.

Concord asked to share millions of pages of evidence turned over by Mueller with Prigozhin, but the documents first have to be reviewed by a "firewall" counsel working for the US government because of Prigozhin's suspected ties to Russian intelligence.

Last month, Concord asked a federal judge to allow discovery regarding how the US government turned over confidential information to the firewall counsel. Prosecutors then asked to file their response under seal on Thursday, citing an ongoing grand-jury case.

Read more: Washington is buzzing about a mysterious grand-jury fight between Mueller's office and an unknown witness

Meanwhile, the grand-jury subpoena case, which has been shrouded in mystery since it was mounted in August, took an interesting turn last month when the DC appeals court issued a ruling revealing that the unnamed witness in the case isn't a person but a foreign corporation, described in documents only as being "owned by Country A."

The ruling indicates that the company has been fighting a subpoena from Mueller's office to hand over information to the grand jury, saying that doing so would violate the law in "Country A."

The document said the court rejected the company's rationale for not complying with the subpoena and ordered it to hand over the information. It also revealed that the corporation is being fined every day that it doesn't comply with the subpoena.

The unnamed company took its case to the Supreme Court after losing at the appeals court, and Chief Justice John Roberts allowed a temporary freeze on the mounting fines last month.

The company and the Justice Department made written arguments to the court last week, and the company submitted a reply under seal to the court on Wednesday. The full court is now gearing up to decide whether to compel the company to pay its fines or to keep the freeze in place.