Paul Kamenar, attorney for Andrew Miller, joined by Peter Flaherty, chairman of the National Legal and Policy Center, left, talks to reporters after a federal judge found Miller in contempt for refusing to testify before the grand jury hearing evidence in special counsel Robert Mueller's investigation of Russian interference in the 2016 presidential election, at the District Court in Washington, Friday, Aug. 10, 2018. (AP Photo/J. Scott Applewhite)

Paul Kamenar, attorney for Andrew Miller, joined by Peter Flaherty, chairman of the National Legal and Policy Center, left, talks to reporters after a federal judge found Miller in contempt for refusing to testify before the grand jury hearing evidence in special counsel Robert Mueller's investigation of Russian interference in the 2016 presidential election, at the District Court in Washington, Friday, Aug. 10, 2018. (AP Photo/J. Scott Applewhite)

WASHINGTON (AP) — An associate of former Trump campaign adviser Roger Stone was held in contempt of court Friday in a fresh attempt to challenge Robert Mueller’s appointment as the special counsel investigating Trump campaign contacts with Russia during the 2016 presidential campaign.

The move also spotlighted a growing focus by Mueller on Stone. Another of Stone’s associates, a New Yorker known as the “Manhattan Madam” because she once operated an upscale escort service, was expected to make her first appearance before a grand jury in the case.

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Paul Kamenar, the attorney for Stone associate Andrew Miller, whose refusal to appear before the grand jury Friday led him to be held in contempt, argued after the proceedings that Mueller’s appointment is unconstitutional. He asserted that Deputy Attorney General Rod Rosenstein did not have the authority to appoint Mueller to lead the investigation into Trump campaign contacts with Russia.

Previous challenges to Mueller’s legitimacy have failed.

President Donald Trump has sought to undermine the investigation by calling it a “witch hunt” and a “hoax.” He has repeatedly insisted, “there was no collusion.”

Kamenar also said a prosecutor with powers as broad as Mueller’s should be treated like a U.S. attorney and be subject to Senate confirmation.

“There’s been no authority, we say, that gives the Justice Department the power to appoint Mr. Mueller,” Kamenar told reporters outside the courthouse in Washington after Miller was held in contempt for refusing to appear before the grand jury.

Kamenar asserted that Mueller has vastly more authority than any U.S. attorney, including the ability to indict foreign actors and bring cases in more than one jurisdiction.

“So our point is if the U.S. attorneys have to be approved by the United States Senate, so, too, should Mr. Mueller,” Kamenar said.

Rosenstein appointed Mueller as special counsel last year after Attorney General Jeff Sessions stepped aside from the Russia investigation. A former U.S. senator, Sessions was one of Trump’s earliest Senate backers and was involved in campaign operations. Sessions met several times with Russia’s ambassador to the U.S.

Mueller has spent months investigating Russian meddling in the election and whether any Trump campaign aides had a hand in the foreign interference plot. A former FBI director, Mueller appears to be putting a focus on Stone, a political consultant and longtime informal adviser to Trump. Mueller’s team has spent months questioning witnesses about Stone, including Stone’s associates.

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An indictment announced last month accused 12 Russian military intelligence officers of hacking into the presidential campaign of Hillary Clinton, Trump’s Democratic opponent, and the Democratic Party, and releasing tens of thousands of private communications in a sweeping Kremlin-orchestrated conspiracy to interfere in the 2016 U.S. election.

According to the charges, the Russian defendants, using a persona known as Guccifer 2.0, in August 2016 contacted a person in touch with the Trump campaign to offer help.

Stone, through his attorney, has acknowledged having a “24-word exchange with someone on Twitter claiming to be Guccifer 2.0.” The statement from lawyer Grant Smith said the exchange “provides no evidence of collaboration or collusion with Guccifer 2.0 or anyone else in the alleged hacking of the DNC emails.”

Miller worked for Stone during Trump’s campaign. Kamenar said his client planned to appeal the contempt ruling in hope that the U.S. Supreme Court will eventually hear the case.

The same grand jury that Miller refused to appear before on Friday was also expected to hear from Kristin Davis, a longtime friend and associate of Stone. He is also godfather to her child. Davis spent several months in a New York City jail for running the escort service.

Miller’s decision to allow himself to be held in contempt is the latest challenge to Mueller’s authority as special counsel.

Paul Manafort, Trump’s former campaign chairman, tried three separate times in federal court to have a judge rule that Mueller lacked the authority to prosecute him. Manafort, who is on currently on trial in federal court in Virginia for alleged financial crimes, lost each time.