Washington(CNN) A defense attorney for Andrew Miller, who's fighting a subpoena from Robert Mueller's investigation into Russian interference in the 2016 election, learned Monday afternoon that the special counsel still wants witness testimony for a federal grand jury.

Paul Kamenar, the defense attorney, says the assertion from Mueller's team made clear to him that Mueller and the Justice Department are considering an additional indictment of Roger Stone or have plans to charge others.

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The development sets up the potential for another twist in the Russia probe. It comes hours after acting Attorney General Matthew Whitaker said that Mueller's investigation was "close to being completed."

Kamenar's client is Miller, a former employee of Stone's whom Mueller subpoenaed in mid-2018 to testify to the grand jury. In a court hearing about Miller's testimony, a judge made clear that Mueller sought information Miller had about Stone's communications regarding Wikileaks and Russian hackers around the time they disseminated damaging hacked Democratic emails.

"The special counsel has advised me the grand jury is still interested in Andrew Miller, and they consider the case still a live case," Kamenar told CNN late Monday afternoon.

Miller faces no criminal charges.

Stone was indicted by the grand jury on Thursday for lying to Congress, witness tampering and obstruction of justice -- but the court papers against him alleged no crimes regarding actual contact between Americans and Julian Assange and the Russians. Stone will be arraigned in Washington Tuesday morning.

A separate but related criminal case against Russian intelligence officers alleges Stone was in contact with them about the hacked documents in 2016, but Stone was not named in that case.

Months ago, Miller spoke to the FBI and turned over digital records to Mueller that he had related to Stone, Wikileaks, Assange, the Democratic National Committee and the online monikers the Russians used after they hacked the Democrats.

When Miller received the subpoena for testimony, he refused to visit the grand jury, and a federal judge held him in contempt of court. The punishment was imprisonment. But Miller hasn't yet been jailed or given the testimony.

Instead, he challenged the subpoena on the grounds that Mueller's appointment as special prosecutor was against the law.

The US Court of Appeals for the DC Circuit heard arguments in the case in early November and has not yet ruled.

Kamenar said Mueller's response to him makes clear the grand jury is still active and at work.

"I can only speculate on why they need my client," Kamenar said.

Kamenar reached out to the special counsel on Friday following Stone's indictment about whether Miller would still be needed.

Peter Carr, a spokesman for the special counsel's office, declined to comment on Monday in response to Kamenar. Attorneys for Stone did not immediately provide a comment.

Justice Department guidelines say that a grand jury can only hear evidence after a person is indicted only if the grand jury continues to work on new charges -- either against that person or against additional planned defendants.

In short, when a defendant like Stone is indicted, prosecutors must already have the evidence needed to take the case they open to trial. So Miller's testimony may relate to a still-unknown criminal case.

In recent months, Mueller's team has brought in other parts of the Justice Department to work on the special counsel's cases, including the National Security Division and the US Attorney's Office in DC. It's possible that if Mueller were to conclude his work, those divisions or others within the Justice Department could take the lead on cases Mueller initially pursued.