Paul Whelan, a U.S. citizen who was arrested in Moscow in 2018 on charges of espionage, has been found guilty in a closed trial and sentenced to 16 years in prison in a case that has strained relations between the two countries.

The verdict was read in a Moscow court on Monday as Whelan stood in the defendant's cage holding a sign that read "Sham trial!"

Secrecy has shrouded the case since the December 2018 arrest of Whelan in his Moscow hotel room. The case of the 50-year-old former U.S. Marine who holds passports from the U.S., Canada, the United Kingdom and Ireland, has become a thorn in the side of U.S.-Russia relations.

Days after his arrest, Russia's Federal Security Service (FSB) said in a statement that he had been detained while on a "spy mission." Later, officials said he was caught with a flash drive containing classified information.

Whelan pleaded not guilty and has denied the charges of espionage, maintaining that he was set up and that he was given a flash drive by an acquaintance that he thought contained family photos.

The prosecution said Whelan held the rank of "at least colonel" with the U.S. Defense Intelligence Agency.

After his arrest, BorgWarner, an American automotive parts supplier, confirmed that Whelan works for the company, serving as its director of global security.

"He is responsible for overseeing security at our facilities in Auburn Hills, Michigan and at other company locations around the world," the company said in a statement to NPR.

U.S. Ambassador to Russia John Sullivan condemned Monday's verdict. An embassy spokeswoman quoted him as saying, "This secret trial in which no evidence was produced is an egregious violation of human rights and international legal norms." Sullivan has also criticized the lack of access to Whelan by embassy staff.

One of Whelan's lawyers, Vladimir Zherebenkov, relayed details of his closed trial, which began on March 23.

According to Zherebenkov, his client called Monday's proceedings "slimy, grubby, greasy Russian politics. Nothing more, nothing less."

Last month, Secretary of State Mike Pompeo tweeted, "We demand Paul's release!"

In June of last year, Whelan appealed to Trump, saying the president could "keep America great" by "aggressively" protecting U.S. citizens such as himself. The White House has been largely silent on the case.

In a statement after the verdict, his brother David Whelan said he believes Monday's conviction increases the chances that Paul will be released as part of a prisoner swap.

NPR's Lucian Kim in Moscow contributed to this report.

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