BRASILIA/MOSCOW (Reuters) - The United States wants an explanation for why Russia detained a former U.S. Marine on spying charges in Moscow and will demand his immediate return if it determines his detention is inappropriate, Secretary of State Mike Pompeo said on Wednesday.

U.S. Ambassador to Russia Jon Huntsman visited the detained man, Paul Whelan, at a detention facility in Moscow and spoke by phone with his family, the State Department said. The United States had expressed concern through diplomatic channels over delayed access to Whelan, who was detained on Friday, a department spokesman said in a statement.

“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 said in Brasilia, where he attended the inauguration of Brazilian President Jair Bolsonaro on Tuesday.

Russia’s FSB state security service opened a criminal case against Whelan but it gave no details of his suspected espionage activities. Under Russian law, espionage can carry a prison sentence of between 10 and 20 years.

Whelan’s family said on Tuesday that he was visiting Moscow for the wedding of a retired Marine and is innocent of the espionage charges against him. He had been staying with the wedding party at Moscow’s Metropol hotel when he went missing, his brother David said.

“His innocence is undoubted and we trust that his rights will be respected,” Whelan’s family said in a statement.

David Whelan told CNN that his brother, who had served in Iraq, has been to Russia many times in the past for both work and personal trips, and had been acting as a tour guide for some of the wedding guests. His friends filed a missing persons report in Moscow after his disappearance, his brother said.

David Whelan did not immediately respond to requests for additional information.

Paul Whelan is 48 and lives in Novi, Michigan, according to public records.

He served in the U.S. Marine Corps Reserves from May 10, 1994 to Dec. 2, 2008 and the highest rank he attained was staff sergeant, according to records provided by the Pentagon. Whelan was discharged for bad conduct following his conviction by court martial on charges related to larceny, the Pentagon said.

BorgWarner Inc BWA.N, a Michigan-based automotive parts supplier, said Whelan is the company’s director of global security and oversees the safety of its facility in Auburn Hills, Michigan, and other locations.

According to the company’s website, it does not have any locations in Russia.

Paul Whelan, a U.S. citizen detained in Russia for suspected spying, appears in a photo provided by the Whelan family on January 1, 2019. Courtesy Whelan Family/Handout via REUTERS

BUTINA CASE

Daniel Hoffman, a former CIA Moscow station chief, said it was “possible, even likely” that Russian President Vladimir Putin had ordered Whelan’s arrest to set up an exchange for Maria Butina, a Russian citizen who pleaded guilty on Dec. 13 to acting as an agent to influence conservative groups in the United States.

Russia says Butina was forced to make a false confession about being a Russian agent.

Putin told U.S. President Donald Trump in a letter on Sunday that Moscow was ready for dialogue on a “wide-ranging agenda,” the Kremlin said, following a series of attempts to schedule a second summit between the leaders.

At the end of November, Trump canceled a planned meeting with Putin on the sidelines of a G20 summit in Argentina, citing tensions about Russian forces opening fire on Ukrainian navy boats and then seizing them.

Trump’s relations with Putin have been under a microscope because of U.S. Special Counsel Robert Mueller’s investigation into suspected Russian meddling in the 2016 U.S. election and possible collusion with the Trump campaign. Moscow has denied interfering in the election. Trump has said there was no collusion and characterized Mueller’s probe as a witch hunt.

Russia’s relations with the United States plummeted when Moscow annexed the Crimean Peninsula from Ukraine in 2014. Washington and Western allies imposed sanctions on Russian officials, companies and banks.