European investigators are focusing on the psychological state of a 27-year-old German co-pilot who prosecutors say deliberately flew a plane carrying 150 people into a mountain, a French police official said Monday.

Returning from a meeting with his counterparts in Germany, judicial police investigator Jean-Pierre Michel told The Associated Press that authorities want to find out “what could have destabilized Andreas Lubitz or driven him to such an act.”

Lubitz was the co-pilot of Germanwings Flight 9525 last week that crashed into a French Alps mountain near Le Vernet en route from Barcelona, Spain, to Duesseldorf, Germany, killing everyone on board.

“To have carried out such an act, it’s clearly psychological,” Michel said.

Authorities are trying to understand what made Lubitz lock his captain out of the cockpit and ignore his pleas to open the door before manually ordering the plane to descend on what should have been a routine flight. To that end, they are speaking with people who knew and worked with Lubitz – such as co-workers, his employer, his doctors.

At the remote mountain crash site itself, French authorities were building a road to facilitate access to the site.

In the southeastern city of Marseille, Germanwings chief operating officer Oliver Wagner was meeting with victims’ relatives. A total of 325 family members have come to France, he told reporters.

French officials have refused to confirm or deny news reports suggesting that Lubitz had been on medication for the treatment of depression or other mental issues. They also refused to comment on a report in Germany’s Bild am Sontag on an alleged transcript of the cockpit voice recorder that had the captain shouting: “For God’s sake, open the door!”

Brice Robin, a state prosecutor in in the southeastern French city of Marseille, has said none of the bodies recovered so far have been identified, denying German media reports that Lubitz’s body had been found.

Tests on the body of the co-pilot may provide clues about any medical treatment he was receiving. German prosecutors said Friday that Lubitz was hiding an illness and sick notes from a doctor for the day of the crash from his employer.

Wagner recalled a meeting in Haltern, Germany, last week with the parents of 16 high school students who had died in the crash, saying it was “certainly the saddest day of my life.”

“They asked `Why our children?'” he said. “We don’t understand what has happened and why it has happened.”

(AP)