Russia formally charged former Marine Paul Whelan with espionage, according to a report Thursday.

“An indictment has been presented. Whelan dismisses it,” Interfax news agency reported, citing a source.

Whelan, a Michigan native, was arrested last Friday by members of the Russian Federal Security Service (FSB) who accused him of being on a “spy mission,” and he has since been detained at Lefortovo Prison in Moscow.

US Ambassador to Russia Jon Huntsman met with Whelan at the prison Wednesday and talked to his family, the State Department said.

Whelan’s family said the 48-year-old, who is director of global security for Michigan-based auto supplier BorgWarner, traveled to Moscow last month to attend the wedding of a fellow Marine veteran to a Russian woman.

Whelan faces up to 20 years in prison if convicted.

The FSB has not released information on why Whelan was arrested.

Secretary of State Mike Pompeo said the US has demanded answers from Russia.

“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,” he said.