The co-pilot of the doomed Germanwings flight deliberately crashed the plane into the French Alps after "rehearsing" the descent on an earlier flight, French investigators said on Wednesday.

Civil aviation investigators said the co-pilot, 27-year-old Andreas Lubitz, had practiced the maneuver on the outbound trip from Dusseldorf to Barcelona just hours before his deadly actions on the return flight.

"Several altitude selections toward 30 meters were recorded during descent on the flight that preceded the accident flight while the co-pilot was alone in the cockpit," read the report. Data from the earlier flight was found in one of two black boxes from the aircraft.