Washington (CNN) George Nader, who was a key witness in the Mueller investigation, pleaded not guilty Friday to campaign finance charges linked to the 2016 election.

He was one of eight people who were recently indicted for allegedly funneling foreign contributions to Hillary Clinton's 2016 presidential campaign and other political groups. One of the other defendants in the case hosted a fundraiser for Clinton, and prosecutors said Nader coordinated more than $3.5 million in illegal donations to gain access to the candidate.

Wearing a green and khaki jail uniform, Nader attended back-to-back hearings at the federal courthouse in Washington, DC. He spoke quietly while answering a few procedural questions from two separate judges, and his hair appeared grayer than in previous appearances.

There's no indication the Clinton campaign was aware of the foreign donor scheme, which was apparently coordinated with at least one Middle East government, according to court filings.

Nader is already in jail , awaiting trial in a separate criminal case in Virginia. He was arrested in June and charged with transporting child pornography. He was previously convicted of sex crimes against minors in the Czech Republic and pleaded guilty to a child porn charge in 1991.

Read More