Andreas Lubitz, 27, who deliberately caused the crash in the French Alps, was being treated by a psychotherapist, Düsseldorf prosecutor’s office spokesman Christoph Kumpa said.
“At that time he was being treated for what is documented as being suicidal [tendencies],” Mr Kumpa added.
He added that Lubitz paid several visits to doctors right up until the time of the crash but none of those visits involved his “suicidal tendencies”.
No suicide note “or anything like that” was found in searches of Lubitz’s German residences, Mr Kumpa said.
There was also nothing in his personal, family or professional background to provide any hints “about his motivation”, Mr Kumpa added.
He also said Lubitz was not suffering from any “organic medical illness”.
Cockpit voice recorder evidence has indicated Lubitz deliberately put the Airbus A320 into a descent after locking out the captain. All 150 people on board, including three Britons, were killed in the crash last Tuesday.
Mr Kumpa was speaking as the grim search for remains continued in southern France.
Lubitz’s girlfriend is believed to have been pregnant with his child and the co-pilot, as well as reportedly having mental health issues, is thought to have been receiving treatment for an unspecified vision problem which could have affected his ability to carry on working as a pilot.
Authorities have already revealed that he hid from his employers a sick note declaring him unfit to work on the day of the disaster, and German newspaper Bild has said he previously told a former girlfriend: “One day I will do something that will change the whole system, and then all will know my name and remember it.”
The Britons killed were Paul Bramley, 28, originally from Hull, Martyn Matthews, 50, from Wolverhampton, and seven-month-old Julian Pracz-Bandres, from Manchester, who died alongside his mother, Marina Bandres Lopez Belio, 37, originally from Spain.
Lufthansa, asked whether the airline had been aware of Lubitz’s previous psychotherapy treatment, said only that all medical information is subject to German medical confidentiality rules.
Prosecutors said they have so far found no indications in Lubitz’s family, his personal surroundings or in his work environment of any motive.
They have not found any sign of a physical illness and have no evidence that he told anyone what he was going to do.
Prosecutors have previously said that, when they searched his home, they found torn-up sick notes from a doctor, one of which would have kept him off work on the day of the crash.
Asked about reports that Lubitz had problems with his vision, Mr Kumpa said there was no evidence showing that Lubitz had any physical ailment affecting his sight.
German aviation officials say Lubitz’s file at the country’s Federal Aviation Office contained a notation that meant he needed “specific regular medical examination”, but it did not specify whether it was for a physical or mental condition.
The US Federal Aviation Administration had issued Lubitz a third-class medical certificate.
In order to obtain such a certificate, a pilot must be cleared of psychological problems including psychosis, bipolar disorder and personality disorders.
The certificate also means he was not found to be suffering from another mental health condition that “makes the person unable to safely perform the duties” of a pilot.
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