The 27-year-old German co-pilot who crashed a Germanwings plane into a remote region of the French Alps last week had been treated for suicidal tendencies years before Tuesday's crash, German prosecutors cited by the Associated Press said Monday. The crash killed all 150 people on board.

Andreas Lubitz had previously seen psychotherapists "over a long period of time" due to "suicidal tendencies," a German prosecutor said in a statement. This treatment reportedly occurred before he received his pilot’s license in 2013. Since then, "no signs of suicidal tendencies or outward aggression were documented," the statement added, and medical records of the pilot did not indicate he had any physical illnesses, the New York Times reported.

In the week since the crash, investigators have scrambled to piece together what drove the co-pilot, Lubitz, to carry out such an act, searching for clues to his past and his psychological history. Based on recordings from the crash, prosecutors determined that Lubitz deliberately flew the plane into a mountain after locking his captain out of the cockpit, but no motive has yet been reported.

"Medical documents were found that indicate an ongoing illness and appropriate medical treatment," an earlier statement from prosecutors in Dusseldorf said, the Guardian reported Friday. That day, investigators had found a torn-up sick note that declared him unfit to work the very day he crashed the Airbus 320 passenger jet, en route from Barcelona, Spain, to Dusseldorf, Germany. "The circumstance that torn-up current medical certificates -- also pertaining to the day of the act -- were found, supports, after preliminary examination, the assumption that the deceased hid his illness from his employer and his professional circles," the statement added.

But the prosecutors have yet to find a suicide note or anything from Lubitz claiming responsibility for the crash. Whether Lubitz had been on medication for depression before the crash or suffered from severe depression early in his life remains unclear.