What goes through the mind of a pilot as he turns the nose of an airliner toward the ground and prepares for death, taking 149 other people with him?
It may be months before investigators delving into the crash of Germanwings Flight 9525 have even a partial answer to that question. A full answer may never be found. A French prosecutor said Thursday that one of the flight's pilots intentionally crashed the plane, adding that the possibility of suicide was "a legitimate question to ask."
But if Andreas Lubitz, the flight's 27-year-old co-pilot, was determined to kill himself, he would not be the first to use an airplane as a tool for self-destruction or to kill passengers and other crew members in the process.
"Aircraft-assisted pilot suicides," as the Federal Aviation Administration calls them, are rare. They include the November 2013 crash of a Mozambique Airlines plane bound for Luanda, Angola, which bears an eerie resemblance to the Germanwings plane's demise. When the flight's co-pilot left to use the lavatory, the captain locked him out of the cockpit and manually steered the aircraft earthward. On the plane's black box recorder, officials said, the co-pilot could be heard knocking on the cockpit door.
The crash of Egypt Airlines Flight 990 off Nantucket, Massachusetts, in 1999, which killed all 217 people on board, was also caused by deliberate action, a National Transportation Safety Board investigation concluded. A relief pilot, Gameel al-Batouti, waited for the captain to leave the cockpit and then disengaged the autopilot. As the plane descended, he could be heard saying in Arabic, "I rely on God," over and over.
Deliberate crashes of commercial airplanes are so rare that researchers have for the most part not been able to study them for patterns. But several studies have looked at general aviation crashes that were caused intentionally, trying to find similarities in the pilots that might help prevent similar suicides in the future.
According to a study conducted for the FAA and published in 2014, out of 2,758 aviation accidents recorded by the agency from 2003 to 2012, only eight were determined to be suicides.
The pilots of all the flights were male, the study found, and ranged in age from 21 to 68. Four had been drinking and two were taking antidepressant drugs at the time of the crash.
The researchers, led by Russell J. Lewis of the agency's Civil Aerospace Medical Institute, found that five of the pilots had given hints of their intentions to others before their final flight.
One of the cases involved a 45-year-old pilot who had a history of depression and at least one suicide attempt that he had not disclosed to the FAA. Witnesses observed the plane heading straight for the ground shortly after takeoff.
In another case, a 47-year-old student pilot who was involved in a custody dispute took off in a Cessna 150 with a child as a passenger. After flying for an hour and half, the plane headed back for the airport but then took a steep dive, crashing into the pilot's ex-mother-in-law's house, according to the FAA study. The pilot and child were killed.
Airline officials have said that Lubitz, who had 630 hours of flight experience, passed the medical and psychological tests required by the airline "with flying colors."
They acknowledged a gap of a few months in his training, but they said that they did not know the reason for the interruption.
Experts on suicide say that the psychology of those who combine suicide with mass murder may differ in significant ways from those who limit themselves to taking their own lives.
"This is not so different in some ways from someone who walks into a school and kills a bunch of people and then kills themselves," said Michelle Cornette, executive director of the American Association of Suicidology.
But she said she could not speculate on Lubitz's case until more details came to light.
Cornette said that it was entirely possible that someone who was suicidal could pass psychological exams and receive a clean bill of health.
"People know what's going to raise a red flag," she said.
Henry Fountain contributed reporting.