DUESSELDORF, Germany » When Andreas Lubitz sent an email in 2009 seeking reinstatement to Lufthansa’s flight-training program after a months-long absence, he appended what in retrospect was a clear warning signal about his fitness to fly passenger jetliners: an acknowledgment that he had suffered from severe depression.

Lufthansa put the young German back through its standard applicant-screening process and medical tests. But it did not, from everything known about the case so far, pursue any plan to ensure that he was getting appropriate treatment. Nor did it impose special monitoring of his condition beyond that required for any pilot who had a flagged health issue. Instead, Lubitz made his way through the training program and was entrusted as an Airbus A320 co-pilot for Lufthansa’s low-cost subsidiary, Germanwings.

The company and its medical staff had no idea of the tortured drama playing out in his mind, peaking in the two or three months leading up to his final flight. After Lubitz flew himself and 149 other people into a French mountainside, Lufthansa’s chief executive pronounced that Lubitz had been "100 percent" fit to fly, showing how unaware the airline was of the extent of the pilot’s psychological troubles.

Lubitz’s journey to the moment when he found himself alone at the controls of Germanwings Flight 9525 from Barcelona, Spain, to Duesseldorf on March 24 exposed a series of failures and weaknesses at Lufthansa and throughout the industry and its regulators in dealing with mental illness among pilots. The pervasive culture of privacy in Germany created a bias against delving into Lubitz’s condition and effectively blinded the country’s airline regulator to the medical problems afflicting German pilots.

The German Federal Aviation Office, which issues pilot’s licenses, relies entirely on the country’s licensed flight doctors to determine pilots’ fitness to fly. But an audit last year by the European Aviation Safety Agency found that Germany’s strict data-protection rules mean that the information flight doctors submit to the regulator is not sufficiently detailed to allow officials to validate the doctors’ findings.

Other nations have taken stricter measures than Germany has when it comes to dealing with depression and other mental illnesses among flight crews.

Dr. Richard Soderberg, chief medical officer at the civil aviation and maritime department of the Swedish Transport Agency, said that while depression would not permanently disqualify a pilot in Sweden, the pilot would be grounded during treatment. The agency would then require the pilot to turn over all medical records pertaining to the depression and submit to psychiatric evaluation every six weeks or so, and the pilot would not be allowed to fly alone.

"The privacy of the pilot cannot be traded for aviation safety," Soderberg said.

Though the highest-profile example of the pilot-suicide problem, Lubitz was far from an isolated case. In recent years, a series of commercial pilots appear to have crashed their aircraft intentionally or been stopped by fellow crew members as they tried. In most cases, those pilots had been screened for psychological problems.

"There was almost a denial by the industry, and in particular among pilots, that we don’t do things like that," said Robert Scott, a former British navy pilot and aviation consultant in Vancouver, British Columbia, who also heads a branch of the Canadian Mental Health Association. "It’s not part of our culture. It’s beyond the pale."

"Now we have so much evidence that we have grudgingly come to accept that, yes, it is a problem," he added.

But flight doctors are well aware of the risks.

"It remains astonishing how many pilots with mild or fading depressive disorders taking antidepressants flew without the knowledge of flight doctors – and still fly for us," Dr. Uwe Stueben and Dr. Juergen Kriebel, who both worked for Lufthansa, wrote in the abstract of a paper published in 2011, while Lubitz was still a trainee.

In 2009, the year Lubitz returned from his months-long absence from training, Stueben was Lufthansa’s director of medical services and would have been involved in evaluating Lubitz’s case, while Kriebel, also employed at Lufthansa’s aeromedical center at the time, normally performed psychiatric examinations on the young trainees.

Stueben declined to comment for this article. Reached by telephone, Kriebel said, "I won’t comment on the situation because the facts of the crash fall under my obligation of confidentiality."

Lubitz applied to join Lufthansa’s flight academy straight out of high school and in 2008 was among the roughly 5 percent of applicants accepted into the training program. But he broke off his training and for several months received psychiatric care. When he was ready to return to the flight school the next year he sent Lufthansa the email about his "episode of severe depression," attaching medical documents, the company said. He received his commercial pilot’s license in 2012.

Before his final flight, Lubitz seems to have methodically plotted his own demise and that of his passengers, investigators say. Although Lubitz had passed his standard medical exam by a flight doctor in August, he had more recent notes from specialists declaring him unfit to work that he never shared with his employer.

Investigators believe he visited many different doctors, in part so none would recognize the true scope of his health problem, according to an official who spoke on the condition of anonymity because of German privacy laws. Prosecutors said Lubitz did not go to Lufthansa for help with his medical condition. That could have grounded him – and possibly cost him his job.

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The jetliner took off from Duesseldorf to Barcelona at 6:45 a.m. on March 24. The captain, Patrick Sondenheimer, flew to Spain but it was agreed that his co-pilot would fly back.

The plane departed from Barcelona shortly after 10 a.m. Before long the plane was cruising at 38,000 feet, the pilots chatting with one another, according to one investigator who has read a transcript of the flight’s cockpit audio recording.

The captain decided to go to the restroom at 10:30 a.m. Lubitz was left alone in the cockpit and he set the aircraft on a gradual but unplanned descent. France’s Bureau of Investigations and Analyses said the readings from the flight’s data recorder showed that Lubitz programmed the autopilot to fly the plane down to an altitude of 100 feet.

Sondenheimer must have realized something was wrong. He pounded on the door with increasing desperation, demanding to be let in. Air traffic controllers urgently called to say the plane’s altitude was too low. Automated systems repeated their commands for him to pull up. From outside the cockpit came screams from the passengers.

Amid this cacophony, Lubitz was silent. Only his steady breathing could be heard on the audio recording as he flew for the last time.

Nicholas Kulish and Nicola Clark, New York Times