Customs agents say they discovered a dolphin skull in luggage left at the Detroit Metropolitan Airport.

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The U.S. Customs and Border Protection announced Friday that agents found the skull last week when they conducted a routine x-ray scan of the luggage, which had been inadvertently separated from owners during transit. The scans revealed the skull of a young dolphin in one of the bags.

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According to the CBP, importing or exporting marine mammals is prohibited. The skull was turned over to U.S. Fish and Wildlife Service investigators.