WASHINGTON (Reuters) - An associate of U.S. President Donald Trump’s longtime adviser Roger Stone was found in contempt on Friday after refusing to appear as a witness before a grand jury convened as part of U.S. Special Counsel Robert Mueller’s Russia probe, U.S. media reported.

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Andrew Miller’s refusal to comply with the subpoena from Mueller’s office prompted a sealed hearing before a federal judge, who then made the ruling, the Washington Post and CNN reported.

In a related development, another Stone associate, comedian and talk show host Randy Credico confirmed that he had been subpoenaed by the special counsel’s office, which is probing alleged Russian interference in the 2016 U.S. presidential election.

Credico told Reuters that he had been summoned to testify before a grand jury on September 7. CNN first reported the subpoena.

CNN also reported that Kristin Davis, another Stone ally and one-time Manhattan madam, was scheduled to appear before the grand jury on Friday.

The latest witnesses in Mueller’s probe appear to suggest it is focusing at least in part on Stone, an early Trump backer with a reputation as an aggressive political operative.

Russia has denied any meddling in the election. Trump has repeatedly said there was no collusion between his campaign and Russia and has called Mueller’s investigation a witch hunt.

Stone, who has denied any wrongdoing, has previously said that eight of his associates have been contacted by Mueller but that the special counsel’s team has not contacted him.

Mueller issued a subpoena to Miller earlier this year, making him the third Stone associate to be identified as having been contacted by Mueller’s investigators.

Miller’s lawyer had sought to quash the subpoena.

That challenge was rejected, however, and U.S. District Chief Judge Beryl Howell on Friday found Miller in contempt but stayed the order pending appeal, the Post reported.

Jason Sullivan, a social media expert who worked with Stone, has testified to the grand jury. John Kakanis, an assistant and driver for Stone, has also been subpoenaed.

Mueller’s outreach may mean he is focusing in part on whether Stone may have had advance knowledge of material detrimental to Trump’s 2016 Democratic opponent Hillary Clinton that U.S. intelligence agencies have concluded was obtained by Russia through hacking.

The U.S. Senate intelligence panel, which is among several congressional committees conducting Russia probes, is also investigating Wikileaks’ role.