Washington (CNN) Robert Mueller's special counsel's office appears to still be investigating Trump adviser Roger Stone and working on a case in court involving one of his former associates.

At least five prosecutors from Mueller's office and two attorneys representing Andrew Miller, a former Roger Stone associate, spent almost 90 minutes in a sealed court proceeding before Chief Judge Beryl Howell of the DC District Court Wednesday morning.

The courtroom activity was not open to the public, but Miller's attorneys previously told CNN they were challenging requests for Miller to provide information to Mueller's grand jury. Their public statements about a challenge to Mueller came before investigators indicted 12 Russian intelligence officers, and Stone surfaced as a person in contact with them during the campaign. Stone was not charged in the Russian hacking indictment Friday.

Yet the court hearing Wednesday following the Russians' indictment signals the grand jury could still be at work.

Howell oversees matters related to the federal grand jury that has indicted several Russians and Americans in Mueller's probe into interference in the 2016 presidential election. She previously ordered an attorney and a real estate agent to testify at the grand jury against Paul Manafort before he was indicted in October.

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