The American man who was arrested last week in Russia on a spying charge is a Marine Corps veteran who was in Moscow to attend a wedding, his family said on Tuesday.

Russia’s Federal Security Service, known as the F.S.B., said on Monday that the American, Paul N. Whelan, had been detained on Friday “during an act of espionage,” and that a criminal case had been opened against him. Conviction on a spying charge in Russia carries a prison sentence of up to 20 years.

“We noticed that he was not in communication,” his parents and siblings said in a statement, “which was very much out of character for him even when he was traveling.”

They said they had not learned of his arrest until it was reported by the news media on Monday. Since then, they have contacted an array of United States government offices.