MOSCOW—Paul Whelan, the U.S. citizen detained by Russian authorities and charged with espionage, could remain in custody for six months or more as the investigation into his case continues, his lawyer said Wednesday.

“Serious, big cases being investigated by the [Federal Security Service] on average last from six months to a year,” Vladimir Zherebenkov told The Wall Street Journal in a telephone interview from the Dominican Republic, where he is vacationing.

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