Federal aviation safety experts say they have discovered a pattern of excessive stress on the wings and tail assemblies of seven Piper Malibu planes that broke up in flight.

Nineteen people - including all seven pilots - were killed in the series of unexplained accidents, which occurred in the United States, Mexico and Japan over a 22-month period beginning in May 1989.The National Transportation Safety Board said Thursday that it is subjecting critical parts of the crashed planes, including their wings, tail assemblies and autopilots, to intensive testing to try to learn the probable cause of the accidents.