An associate of former Trump campaign adviser Roger Stone has been held in contempt of court after refusing to appear before a grand jury in the special counsel’s Russia investigation. His lawyer says he will challenge the constitutionality of Robert Mueller’s appointment as special counsel.

Paul Kamenar, an attorney for Andrew Miller, says Mueller’s appointment is unconstitutional, contending Deputy Attorney General Rod Rosenstein didn’t possess the authority to appoint a special counsel and that any prosecutor with Mueller’s broad powers should be confirmed by the Senate. Rosenstein appointed Mueller after Attorney General Jeff Sessions recused himself from the investigation. Miller refused to appear before a grand jury Friday in Washington.

Andrew Miller, a media liaison for Stone during the 2016 presidential election, was issued a subpoena in June to appear before a grand jury and hand over documents to Mueller’s office.

In addition to Miller, John Kakanis, described as a personal assistant to Stone, along with social media expert Jason Sullivan, were subpoenaed by the special counsel over the spring.

FBI agents grilled Kakanis over Roger Stone’s contact with WikiLeaks editor-in-chief Julian Assange and hacker Gufficer 2.0 during the 2016 presidential election.

In May, Stone told NBC host Chuck Todd that he anticipates being indicted by the Office of the Special Counsel.

“I can guarantee you they have found no evidence whatsoever of Russian collusion, nor trafficking of allegedly hacked emails with WikiLeaks,” said Stone. “It is not inconceivable now that Mr. Mueller and his team may seek to conjure up some extraneous crime pertaining to my business, or maybe not even pertaining to the 2016 election.”

“But I think it just demonstrates, again, this was supposed to be about Russian collusion, and it appears to be an effort to silence or punish the president’s supporters and his advocates,” the veteran political operative added.

Appearing before the House Permanent Select Committee on Intelligence last fall, Sone told lawmakers he was “aware of no evidence whatsoever of collusion by the Russian state or anyone in the Trump campaign.”

The Associated Press contributed to this report.