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By Eveline Danubrata and Nguyen Phuong Linh KUALA LUMPUR/HO CHI MINH CITY, Vietnam (Reuters) - U .S. investigators probing the disappearance of a Malaysia Airlines passenger jet believe it may have flown for four hours after losing contact with air traffic controllers, The Wall Street Journal reported on Thursday. If confirmed, the report would represent another dramatic twist in what is already one of the most baffling mysteries in modern aviation history - the fate of Flight MH370, which took off from Kuala Lumpur early on Saturday and dropped off civilian radar screens less than an hour into its flight to Beijing. The Wall Street Journal said U.S. aviation investigators and national security officials believed the plane flew for a total of five hours, based on data automatically downloaded and sent to the ground from the Boeing 777's engines as part of a standard monitoring program. A senior Malaysia Airlines official told Reuters that no such data existed, while a second official said he was unaware of it.