Kim Sang Dok, who taught accounting at a private Pyongyang university, was ‘intercepted’ last month

Kim Sang Dok, a U.S. citizen detained last month by North Korea, taught at Pyongyang University of Science and Technology, a private school founded by a Korean-American businessman.

SEOULA 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.

His wife, who was with him in Pyongyang, returned to the U.S. shortly after his detention, according to a person familiar with the matter.

Mr. Kim, also known by his English name, Tony, taught at Pyongyang University of Science and Technology. The school, founded by a Korean-American businessman, said last month that Mr. Kim had been detained as part of an investigation into “matters that are not connected in any way with the work of PUST,” and declined to comment on any of his activities beyond his teaching work. According to the KCNA, his subject was accounting.

South Korea’s Yonhap News Agency reported last month that Mr. Kim is in his late 50s and had been involved in aid work in North Korea. It said he was in the country to discuss relief activities.

Mr. Kim is the third known U.S. citizen to be detained by North Korea in recent months. Pyongyang last year sentenced Otto Warmbier, a University of Virginia undergraduate arrested for allegedly trying to steal a political poster from a hotel, and Kim Dong-chul, a Korean-American businessman, to terms of 15 years and 10 years of hard labor, respectively.

A U.S. State Department spokesman last month declined to comment on Tony Kim’s case, citing privacy concerns, but said the department typically works with the Swedish Embassy in Pyongyang when U.S. citizens are detained.

Representatives for the U.S. Embassy in Seoul didn’t immediately respond Wednesday to requests for comment.

Many of the humanitarian groups with U.S. ties that do aid work in North Korea are associated with Christian organizations. PUST’s founder is a Christian, as are many members of the teaching staff. North Korea views the spread of organized religion as a threat to the ruling family’s grip on power. In 2014 American Jeffery Fowle was detained and held for six months after leaving a Bible in a nightclub bathroom.

In the past, high-profile U.S. envoys have been dispatched to North Korea to secure the release of U.S. citizens. Laura Ling and Euna Lee, two journalists sentenced to 12 years’ hard labor for illegally entering the country in 2009, were released later that year when former President Bill Clinton flew in and met with then-leader Kim Jong Il. Kenneth Bae, a Korean-American missionary sentenced to 12 years’ hard labor for “hostile acts,” was freed in November 2014—after more than two years’ detention—when James Clapper, the U.S. director of national Intelligence, traveled to Pyongyang.

Tensions between Pyongyang and Washington have grown tauter in recent months. In September North Korea conducted its fifth nuclear test and last month it paraded a new long-range missile, a day before a failed missile test. On Tuesday, North Korea complained about two U.S. bombers that conducted flyovers near the Korean Peninsula.

Separately on Tuesday, the U.S. declared operational an advanced U.S. missile-defense system being installed in South Korea. On Wednesday, KCNA said the consequences will be “miserable.”

“South Korea can suffer great disaster any moment,” it said, without elaborating.