A U.S. citizen detained in Russia on suspicion of espionage was in Moscow for a wedding, his twin brother said Tuesday, expressing concern about his safety.

David Whelan, whose brother Paul Whelan was arrested on Friday, told CNN that their family grew worried when he didn't show up for a wedding ceremony between a fellow ex-Marine and a Russian woman.

"We are deeply concerned for his safety and well-being. His innocence is undoubted and we trust that his rights will be respected," David Whelan told CNN in a statement.

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David Whelan said his brother flew to Moscow on Dec. 22. He was reportedly detained on Dec. 28, and the family learned of his arrest through media reports, his brother said.

"When we couldn't get ahold of him initially, we were worried, and we are still worried now, but at least we know he is alive," he told CNN.

The Federal Security Service of Russia said Monday that its officers had detained Paul Whelan during a spy operation in Moscow, but did not offer details on his alleged misdeeds.

The spying charges leveled against Paul Whelan could carry a prison sentence of up to 20 years, The Associated Press reported.

Paul Whelan's arrest comes amid strained relations between the U.S. and Russia.

Maria Butina, a Russian woman arrested in the U.S. earlier this year, pleaded guilty in December to acting as an unregistered agent of the Russian government in the U.S.

She faces a maximum of five years in prison. Russia has called for her release.