CANBERRA, Australia—Examination of wing-flap debris from the missing Malaysia Airlines Flight 370 shows the aircraft was most likely in a steep dive when it plunged into the ocean, reinforcing the long-held theory that the plane wasn’t under human control at the time of its apparent crash in March 2014.

Australian investigators overseeing the search for the airliner in a remote area of the Indian Ocean released a report Wednesday saying the wing flap likely wasn’t extended when the plane hit the water, suggesting the aircraft...