A court in Russia has denied bail to an American man being held on spying charges.

Paul Whelan, a dual U.S.-British citizen and former U.S. Marine, was arrested December 28 for what the Russian Federal Security Service (FSB) said was “carrying out an act of espionage,” without specifying exactly what he was accused of doing.

His family has said he is innocent.

The 48-year-old appeared in the Moscow court Tuesday seeking to be released from custody while his case goes forward, but the judge did not approve the request.

There is speculation the Russians might try to use Whelan as leverage for the release of Russia’s Maria Butina, who pleaded guilty to infiltrating America’s conservative political movement as a Kremlin-directed agent. Butina was convicted of acting as a foreign agent in the United States.

The Kremlin has dismissed the idea of Whelan being held for a possible prisoner swap.