A co-pilot of the Germanwings plane which crashed Tuesday killing 150 people, deliberately flew the plane to its fatal ending on the mountaintops in the French Alps, a prosecutor heading the criminal investigation said Thursday.

The revelation came after the prosecutor established from flight recording that one of the pilots was locked out of the cockpit before it crashed.

The pilot had left the cockpit, and was disallowed from entering, the prosecutor, Brice Robin, said.

The co-pilot then began the plane’s descent for an unknown reason while he was alone in the cockpit, Mr. Robin said.

The Airbus A320 operated by Germanwings was traveling to Düsseldorf, Germany, from Barcelona, Spain.