Moscow (CBS) - A Russian court found former U.S. Marine Paul Whelan guilty of espionage on Monday and sentenced him to 16 years in prison. For his family, it was a predictable conclusion to the high-profile spy case that has been yet another strain on relations between Moscow and Washington.

Whelan, 50, was arrested at Moscow's Metropol Hotel in December 2018. Investigators claim he was caught red-handed after receiving a USB drive containing classified information.

The U.S., who also holds British, Irish and Canadian citizenship, has maintained throughout the trial that he was framed by an agent from Russia's Federal Security Service (FSB), who gave him the drive. He pleaded not guilty during the trial and insisted he was not an U.S. spy. Prosecutors had sought an 18-year prison sentence in the case. The charges carried a possible maximum sentence of 20 years. U.S. intelligence and State Department sources had told CBS News previously that they were confident Whelan wasn't a spy.

While most of the charges against the U.S. have remain sealed, Russian media reports said Whelan stood accused of trying to recruit a long-time Russian acquaintance to gain a list of names of employees of a Russian security agency.

All of the hearings in the trial before the reading of Monday's verdict were held behind closed doors, under strict restrictions implemented to control the spread of the coronavirus in Moscow. Whelan's family had expressed hope that the end of the trial might mean the U.S. could push harder for his return from Russia.

"We hope that the U.S. government and the Russian Federation will begin discussing Paul's release immediately, now that there will no longer be any procedural impediments," Whelan's brother David said in a statement emailed to CBS News ahead of the sentencing last week.

The Russian Foreign Ministry has insisted that Moscow could only discuss a prisoner exchange once a person is formally convicted of a crime. U.S. Embassy in Russia condemned the prosecution of Whelan and criticized the trial for its lack of transparency.

In a tweet sent by the U.S. Embassy's spokesperson, Ambassador John Sullivan criticized Russia for the "secret trial."

"Fair and transparent? No. Evidence produced? No. The world is watching," the Ambassador said.

Speaking about the case last week, Secretary of State Mike Pompeo reiterated the Trump administration's long-standing demand for Whelan's release.

At the time of his arrest, Whelan was the director of global security for Michigan-based auto parts supplier BorgWarner. Before that he spent 14 years in the U.S. Marine Corps before being discharged in 2008 for bad conduct, according to the military. He served in Iraq for several months in 2004 and 2006.

Whelan had visited Russia several times before his arrest in 2018. Last month he underwent an emergency hernia surgery at a Moscow hospital.