MOSCOW—A former U.S. Marine detained in Russia was found guilty and sentenced to 16 years in prison for espionage in a case that has drawn condemnation from the U.S. and threatens to further erode troubled relations between Moscow and Washington.

A Moscow City Court judge ruled Monday that Paul Whelan, from Novi, Mich., who also holds U.K., Canadian and Irish citizenship, was guilty of spying and should serve the sentence in a high-security penitentiary.

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