KUALA LUMPUR, Malaysia—A search-and-rescue plane spotted suspected fragments of a missing Malaysian airliner in the first potential breakthrough in the investigation of what happened to the flight after it disappeared early Saturday morning.

The fragments were believed to be a composite inner door and a piece of the plane's tail, Vietnam's ministry of information and communication said on its website. The objects were located about 50 miles south-southwest of Tho Chu island.

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