The Russian Federation formally charged US citizen Paul Whelan with espionage Thursday, according to Russian news agency Interfax.

Russia’s Federal Security Service (FSB) arrested Whelan on December 28.

Under Article 276 of the Criminal Code of the Russian Federation, espionage is punishable by 10-20 years in prison. The family of Whelan, a retired US Marine, claim that he was in Moscow to attend a wedding.

US Secretary of State Mike Pompeo, speaking from Brazil, said:

With respect to Mr. Whelan, who is being held, we are hopeful within the next hours we will get consular access to see him and get a chance to learn more. 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.

The Ministry of Foreign Affairs of the Russian Federation allowed US Ambassador to Russia Jon Huntsman to meet with Whelan.

The arrest comes amid heightened tensions between the US and Russia. Recently, Maria Butina pleaded guilty to conspiracy to act as an illegal foreign agent in United States District Court for the District of Columbia.