WASHINGTON -- 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, attorney for Andrew Miller, says Mueller's appointment is unconstitutional. He argues that Deputy Attorney General Rod Rosenstein didn't have 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 worked for Stone during Donald Trump's 2016 presidential campaign.

Miller refused to appear before a grand jury Friday in Washington.

Miller plans to appeal.