BANGKOK, THAILAND—The U.S. government has returned hundreds of artifacts to Thailand in a ceremony at the country’s National Museum. The artifacts were recovered from a museum in southern California after a five-year, undercover federal investigation. Many of them had been looted from Ban Chiang, a UNESCO World Heritage site, in the 1970s. “Of the artifacts returned to Thailand, we can say that the 554 pieces, most of them are priceless because they are dated to a prehistoric period,” Vira Rojposhanarat, the kingdom’s culture minister, told Voice of America. Rojpochanarat accepted the artifacts from U.S. Charge D’Affaires W. Patrick Murphy, the highest-ranking American diplomat in the country.