MOSCOW -- The lawyer for an American man being held in Moscow on suspicion of spying said Tuesday that classified Russian materials were found on him when he was arrested.

Paul Whelan, a former U.S. Marine, was arrested in Moscow at the end of December. The arrest raised speculation that he could be swapped for one of the Russians held in the U.S., such as gun rights activist Maria Butina, who has pleaded guilty to acting as a foreign agent in the U.S.

Whelan is to appear in court on Tuesday where the judge is expected to extend his arrest.

Spying charges carry a prison sentence of up to 20 years in Russia.

Whelan's lawyer, Vladimir Zherebenkov, told Russian news agencies before the hearing on Tuesday that when Whelan was detained at a Moscow hotel at the end of December he had something with him that contained "state secrets."

The lawyer added that he did not know how Whelan got hold of those materials or what he was going to do with them.