SEOUL—A U.S. citizen detained last month in North Korea is being investigated for seeking to overthrow the government, the country’s state-controlled news agency said.

Kim Sang Dok committed “criminal acts of hostility aimed to overturn” North Korea—“not only in the past but also during his last stay before interception”— the Korean Central News Agency said Wednesday, adding that the “interception” had taken place April 22 at Pyongyang’s international airport.

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