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.

Remi Jouty, director of the BEA, the French authority responsible for safety investigations into civil aviation accidents, said there had been no noticeable effect as the plane had already begun its descent into Barcelona while Lubitz was carrying out the practice manipulations.

The Germanwings Airbus 320 crashed in the French Alps on March 24, killing all 150 people on board.

The investigation into the crash confirmed initial suspicions that Lubitz deliberately brought the plane down.

On the fatal flight back to Dusseldorf, which left at 9am local time, everything initially proceeded normally, with Lubitz even eating his meal 15 minutes into the flight, the report said.

At 9.30 am the captain left the cockpit to go to the toilet, and the selected altitude on the flight control unit changed "in one second" from cruising altitude of 11,600 meters to 30 meters - the minimum height possible to select on an A320.

The speed of the plane was also gradually increased.

During the descent, air traffic controllers in Marseille called the plane 11 times on three different frequencies but received no response.

The air force also tried to contact the plane three times.

The cockpit recorder showed the pilot's frantic efforts to re-enter the cockpit. The interphone rang three times and he triggered a signal in the cockpit when he entered a code to open the door, which is heavily reinforced to prevent hijackings.

A person was heard knocking on the door six times and a muffled voice could be heard asking for the door to be opened.

Another five "noises similar to violent blows" could be heard just two minutes before the end of the flight and a warning system blaring "Terrain, Terrain, Pull Up, Pull Up" sounded until the crash.

Worrying diagnosis

German prosecutors have said Lubitz was diagnosed as suicidal several years ago before he became a pilot but had appeared more stable of late.

Doctors had recently found no signs that he intended to hurt himself or others, but he was receiving treatment from neurologists and psychiatrists who had signed him off sick from work several times, including the day of the crash.

Police found torn-up sick notes during a search of his apartment after the crash.

The report said Lubitz had been issued with a yearlong medical certificate without restrictions in April 2008, which was not renewed for four months due to a course of antidepressant medication.