The entity that is the subject of the cloaked legal battle — known in court papers simply as a “Corporation” from “Country A” — is a foreign financial institution that was issued a subpoena by a grand jury hearing evidence in the special counsel investigation, according to two people familiar with the case.

It is thought to be the first time that an aspect of Mueller’s wide-ranging probe into Russian interference in the 2016 campaign has reached the Supreme Court.

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Earlier Tuesday, the corporation’s lawyers indicated that they plan to continue fighting the subpoena, asking the Supreme Court to review the merits of the decision by the U.S. Court of Appeals for the D.C. Circuit.

Last year, a federal court in Washington ordered the corporation to pay a daily fine of $50,000 until it complied with the subpoena, according to court records. An appeals court panel upheld that decision last month.

In a separate redacted opinion published Tuesday, the D.C. Circuit reiterated its reasoning, writing that even though the subpoena targets a foreign entity, “there is a reasonable probability the information sought through the subpoena here concerns a commercial activity that caused a direct effect in the United States.”

The appeals court concluded that the foreign company is not immune from the reach of a U.S. grand jury.

Because the company “failed to satisfy its burden of showing that Country A’s law would prohibit complying with the subpoena, we agree with the district court that enforcing the subpoena is neither unreasonable nor oppressive,” according to the order from Judges David Tatel and Thomas Griffith. The third judge on the panel, Stephen Williams, wrote a separate opinion agreeing with the order.

The company’s lawyers appealed the decision to the Supreme Court. Late last month, Roberts, who receives emergency petitions from the U.S. Court of Appeals for the D.C. Circuit, put the order and the fines on hold until the justices could consider the matter.

The secret nature of the case has prompted a Washington guessing game about the information that Mueller is pursuing.

In the lower courts and at the Supreme Court, details of the legal battle to get information from the corporation have been sealed. At one point, an entire floor of the D.C. Circuit was closed to the public to protect the identity of the litigants.

A panel of the D.C. Circuit solved at least part of the mystery when it released its vaguely worded decision on Dec. 18.

“The grand jury seeks information from a corporation owned by Country A,” the three-page opinion stated. The company had sought to quash the subpoena by arguing it is protected from such demands under the Foreign Sovereign Immunities Act and that complying with the subpoena would cause the company to violate its own domestic laws, according to the opinion.

In its ruling, the appeals court was particularly dismissive of the claims that the unidentified country’s domestic laws barred compliance with the subpoena. “We are unconvinced that Country A’s law truly prohibits the corporation from complying with the subpoena,” the court wrote.

At the Supreme Court, the case is simply titled In re Grand Jury Subpoena.