The head of Lufthansa said Thursday there were no clues to what led the co-pilot of a jet operated by its subsidiary Germanwings to deliberately crash the plane.

Carsten Spohr, chief executive of the German carrier, said he was "stunned" by findings that 28-year-old Andreas Lubitz appeared to have deliberately slammed the Airbus A320 into the French Alps.

All 150 people on board were killed in Tuesday's crash.

He told a news conference shortly after French officials gave an account of the flight's last minutes based on initial information gleaned from cockpit recordings that there was "no indication what might have led" to the young co-pilot's actions.

French Alps
Lufthansa employees pay a minute of silence in Frankfurt, Germany, Wednesday, March 25, 2015, for the 150 victims who died in the Germanwings plane crash in the Frenchalpson the way from Barcelona to Duesseldorf on Tuesday. - AP Photo/Michael Probst
"In our worst nightmares we could not have imagined that this kind of tragedy could happen to us here at the company," Spohr said.

Cockpit employees are selected "very, very carefully" with much attention paid to their "psychological suitability", the Lufthansa chief, himself a former pilot, added.

Lubitzbegan his training in 2008 and six years ago had taken a break during theprogrammebefore resuming again. He had passed his medical, flying and other tests and checks.

A Lufthansa spokeswoman said he had 630 hours of flight experience.

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"He was 100 percent airworthy, without reservation," Spohr said.

He added that no security "system in the world" could protect an airline from such a disaster.

"Whatever safety provisions you haveina company, however high the standards, such an isolated case cannot be completely ruled out," Spohr told reporters.

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