Russia has formally charged U.S. citizen Paul Whelan with espionage, Moscow’s Interfax news agency reported on Thursday.

“An indictment has been presented. Whelan dismisses it,” an unnamed source told the news organization.

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Russia’s Federal Security Service, the FSB, revealed earlier this week that Whelan had been detained in Moscow on Dec. 28 on accusations of spying. Thursday’s reports are the first news of him being formally charged.

Whelan, a former Marine and security executive from Michigan, had been traveling to Russia for a wedding, according to members of his family. They have fiercely pushed back on Moscow’s allegations that he was conducting espionage.

“I just can’t see him breaking laws in America, let alone going to a country that might be more difficult to navigate if he broke the law, and certainly not breaking a law of espionage,” Whelan’s brother David told the Detroit Free Press this week.

U.S. Ambassador to Russia Jon Huntsman visited Whelan at the Lefortovo Detention Facility in Moscow on Wednesday, according to the State Department. Huntsman has also been in touch with Whelan’s family members in the United States.

Meanwhile, Secretary of State Mike Pompeo Michael (Mike) Richard PompeoThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep Trump's foreign policy successes confound his detractors It's time for a Jackson-Vanik Amendment for China MORE on Wednesday called for more information from the Kremlin about the charges and said the Trump administration would “demand” Whelan’s immediate return if they discover that the accusations are unfounded.

“We’ve made clear to the Russians our expectation that we will learn more about the charges, come to understand what it is he’s been accused of, and if the detention is not appropriate, we will demand his immediate return,” Pompeo told reporters while on an official trip in Brazil.

Whelan's arrest came weeks after Maria Butina pleaded guilty in U.S. federal court to acting as a foreign agent for Russia in the United States, fueling speculation that the Kremlin detained him in an act of retaliation and could look to exchange him for Butina. Russia has denied knowledge of Butina or her efforts.

Whelan's lawyer, Vladimir Zherebenkov, told the New York Times Thursday that he would welcome an exchange but noted it would require Whelan be pardoned by Russian President Vladimir Putin.

"This is a long process,"Zherebenkov said. "I myself hope that we can rescue and bring home one Russian soul."

Whelan works for automotive parts maker BorgWarner in Michigan. His family said they first learned of his arrest from news reports on Monday, when the FSB revealed he had been apprehended.

Whelan could face up to 20 years in prison if convicted.