Senior executives of Lufthansa, Germanwings' parent company, visited the crash site of Flight 9525 in the French Alps on Wednesday and paid their tribute to the 150 people who died last Tuesday. The visit came amid mounting questions about the psychological condition of the plane’s co-pilot, who is alleged to have deliberately caused the crash.

Lufthansa CEO Carsten Spohr, who visited the crash site in Seyne-les-Alpes along with the head of its low-cost airline Germanwings, Thomas Winkelmann, said that it will take "it will take a long, long time to understand how this could happen.” However, he refused to comment on co-pilot Andreas Lubitz's psychological state, the Associated Press (AP) reported. Spohr reportedly said that the airline is "learning more every day" about what might have led to the crash.

So far, the analysis of the plane's voice data recorder has revealed that Lubitz initiated the plane’s descent and deliberately crashed it. Lufthansa had acknowledged Tuesday that Lubitz experienced a “deep depressive episode” while he was attending flight-training school in 2009, but that he had passed all his medical checks since.

German prosecutors said that Lubitz had previously seen psychotherapists "over a long period of time" due to "suicidal tendencies," before he received his pilot’s license in 2013.

Investigators, who are trying to determine why Lubitz might have intentionally caused the crash, have not found any suicide note in Lubitz’s apartment in Dusseldorf or his parents' house in Montabaur.

Meanwhile, on Wednesday, authorities said that "there are no longer any visible remains" at the crash site, AP reported