TOKYO—Japan sent an official delegation to North Korea on Monday to examine progress on Pyongyang’s investigation into the fates of Japanese nationals the North abducted decades ago.

The trip, the first of its kind in 10 years, comes after an expected preliminary report on the investigation was delayed, raising skepticism in Japan about whether Pyongyang will follow through on its pledge to provide information on Japanese citizens abducted in the 1970s and ’80s to train North Korean spies.

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