Russia has charged a retired U.S. Marine arrested in Moscow with espionage, the Interfax news agency reported, meaning he could face up to 20 years in prison if found guilty.

Russia's Federal Security Service (FSB) said on December 31 that it had arrested Paul Whelan, who it said was caught carrying out an act of espionage.

Interfax said the Russian authorities had brought formal charges against Whelan on January 3.

Whelan's lawyer, Vladimir Zherebenkov, did not comment on the charges but said that under the terms of his arrest, Whelan was expected to remain in pretrial custody in Moscow until at least February 28.

Zherebenkov said on January 3 that he had appealed the two-month pretrial custody order, asking for Whelan to be released on bail instead.

“I consider his detention and arrest baseless,” Zherebenkov told Reuters. “It's based on investigators' supposition that he will escape and hinder the investigation process. We are asking for bail instead."

Zherebenkov told Reuters that Whelan, who has denied the charges, is “feeling well, fresh, confident” and “all his rights and all international principles are being fully followed.”

“I presume that he is innocent because for now I haven’t seen any evidence against him that would prove otherwise,” Zherebenkov told The New York Times on January 3. “He wanted to learn more about Russia. He was interested in the country, its culture, and the Orthodox faith."

The FSB has not disclosed the nature of Whelan’s alleged espionage activities.

U.S. Secretary of State Mike Pompeo said on January 2 that the United States was seeking an explanation for why Russia arrested Whelan on spying charges and will demand his immediate return if it determines his detention is inappropriate.

The U.S. State Department said U.S. Ambassador to Russia Jon Huntsman visited Whelan at a detention facility in Moscow on January 2 and spoke by phone with his family.

Whelan's family has denied he was a spy and said he was in Moscow to attend a wedding.

The family said Whelan was last heard from on December 28.

Whelan, 48, is a former U.S. Marine who lives in the state of Michigan and is director of global security at BorgWarner, a U.S.-based supplier of automotive parts and components.

Relations between Washington and Moscow are badly strained over issues including Russia’s role in wars in Syria and eastern Ukraine, its alleged meddling in elections in the United States and elsewhere, and the poisoning of a double agent in Britain.

The detention of Whelan comes weeks after Russian Maria Butina pleaded guilty in a U.S. court to acting as an agent for the Kremlin.

The Kremlin has denied that Butina is a Russian agent and has organized a social-media campaign to secure her release.

In the past, Russia has sometimes arrested foreigners with the aim of trading prisoners with other countries.

With reporting by Reuters, AFP, and Interfax