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Mr. Richardson, a former U.S. ambassador to the United Nations who often serves as an envoy to countries that do not have diplomatic relations with the United States, will try to meet with North Korean officials and possibly Mr. Bae to discuss the case, the sources said.

Mr. Richardson has been to North Korea at least a half-dozen times since 1994, including two trips to negotiate the release of Americans detained by North Korea. His last visit to Pyongyang was in 2010.