National Transportation Safety Board Chairman Deborah Hersman has said investigators initially found no mechanical problems with the plane, but were investigating further.
According to the expert, who wasn't authorized to talk publicly about the investigation and spoke only if not named, there have been previous incidents in which automatic throttles on some Boeing planes have disengaged for no known reason. The auto-throttle controls engine power and thus speed.
The possible equipment malfunction was first reported by The Wall Street Journal.
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