MOSCOW - Paul Whelan, a former U.S. Marine detained in Moscow on spy charges, was given a flash drive containing a "state secret," his lawyer said on Tuesday, delivering the first official details in a mysterious case that has been wrapped in silence for weeks.

Whelan, 48, who also holds British, Canadian and Irish citizenships, was arrested by Russian security services in late December, during what his family said was a personal trip to the Russian capital, and later charged with the relatively broad crime of espionage.

Speaking in a Moscow court, his lawyer Vladimir Zherebenkov told reporters, "how he got [the flash drive], what he was supposed to do with it, and whether Whelan knew that he had secret information is unknown."

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Zherebenkov said Whelan, a frequent visitor to Russia over the last decade, expected the flash drive to contain photographs and videos of one of his previous trips to the country. He had asked an acquaintance to send him the information as he had been unable to download it electronically.

"He only spoke to regular people in Russia," Zherebenkov added.

At the closed-door, pretrial hearing, the Russian court refused to release Whelan on bail, meaning he will remain behind bars through the end of February.

Wearing glasses and a blue button-down shirt, the Michigan resident was held in a metal and glass enclosure where he looked on, concerned, as he spoke to his lawyer through an interpreter.

Whelan spoke in court for 15 minutes and gave a "detailed description of his response to the charges," Zherebenkov said, adding that Whelan maintained his innocence. Zherebenkov reiterated Whelan's family's claim that he was in Moscow for the wedding of a friend before being seized in an upscale hotel room, a stone's throw from the Kremlin.

The case of Whelan, who was court-martialed for stealing thousands of dollars from the U.S. government and dishonorably discharged, has mystified reporters and officials alike in Russia.

Former U.S. intelligence officials have said that Whelan's background, and the fact he was not in Russia under diplomatic cover, make him an unlikely spy. This led some experts to speculate he could be traded for a Russian asset in the United States, such as gun rights activist Maria Butina.

Butina, 30, pleaded guilty last month to trying to influence the 2016 presidential election.

Zherebenkov stressed on Tuesday that no swaps could take place before a conviction.

The hearing was attended by consular officials from all four countries, representing a first for Britain and Canada, who have been denied access to Whelan while in prison.