Missing plane mystery: Pilot suicide a taboo topic in past crash probes

Egyptian officials, meanwhile, rejected the notion of suicide altogether, insisting instead there was some mechanical reason for the crash.

There was also disagreement over the cause of the crash of SilkAir Flight 185, which plunged into a river in 1997 during a flight from Jakarta, Indonesia, to Singapore, killing all 104 passengers and crew.

A U.S. investigation found that the Boeing 737 had been deliberately crashed, but an Indonesian investigation was inconclusive.

Mozambique officials have been investigating a crash that killed 33 people in November.

They say preliminary investigations indicate that the pilot of the Mozambican Airline plane bound for Angola intentionally brought it down, and they're continuing to look into his possible motives.

A 2014 study by the Federal Aviation Administration indicates that in the U.S. at least, flying remains a remarkably safe mode of transport and pilot suicide is a rare occurrence.