The hunt is on for second data recorder as police search the house of 28-year-old Andreas Lubitz who officials say set Germanwings flight on its fatal descent

A young German co-pilot locked himself in the cockpit of Germanwings flight 9525 and flew it into a mountain, killing all 150 people on board including himself, prosecutors said yesterday.

After listening to black box voice recordings, French prosecutors left no doubt that they believe 28-year-old Andreas Lubitz was in control of the Airbus A320 and set it on its fatal descent. They offered no explanation for his motive.

German co-pilot Andreas Lubitz.

Prosecutors in the German city of Düsseldorf said police were searching his home for evidence.

French and German officials said there was no indication Tuesday’s crash in the French Alps was a terrorist attack. Acquaintences described Lubitz as an affable young man who had given no sign of harbouring harmful intent.

According to Marseille prosecutor Brice Robin, Lubitz acted “for a reason we cannot fathom right now but which looks like intent to destroy this aircraft”.

“He voluntarily... allowed the loss of altitude of the plane, which he had no reason to do. He had ... no reason to stop the pilot-in-command from coming back into the cockpit. He had no reason to refuse to answer to the air controller who was alerting him on the loss of altitude,” Robin said.

Only towards the end do you hear screams

The captain, who had stepped out of the cockpit, probably to use the toilet, tried to force his way back in: “You can hear banging to try to smash the door down,” Robin said.

Describing sound recordings from one of the plane’s black boxes, Robin said most of the passengers would not have been aware of their fate until the very end.

“Only towards the end do you hear screams,” he said. “And bear in mind that death would have been instantaneous... the aircraft was literally smashed to bits.”

French gendarmes and investigators, seen in this picture released by the French Interior Ministry.

German police officers leave the house believed to belong to the parents of crashed Germanwings flight 4U 9524 co-pilot Andreas Lubitz in Montabaur.

FlightRadar24, an online air tracking service that uses satellite data, said it had found evidence the autopilot was abruptly switched from cruising height to maintain an altitude of just 100 feet, the lowest possible setting. The plane crashed at about 6,000 feet.

'Between 09:30:52 and 09:30:55 you can see that the autopilot was manually changed from 38,000 feet to 100 feet and 9 seconds later the aircraft started to descend, probably with the 'open descent' autopilot setting,' Fredrik Lindahl, chief executive of the Swedish tracking service, said.

The CEO of Lufthansa, parent company of Germanwings, said its air crew were picked carefully and subjected to psychological vetting.

“No matter your safety regulations, no matter how high you set the bar, and we have incredibly high standards, there is no way to rule out such an event,” CEO Carsten Spohr said. The world’s attention will now focus on the motivations of Lubitz, a German national who joined the budget carrier in September 2013 and had just 630 hours of flying time – compared with the 6,000 hours of the flight captain, named in German media only as ‘Patrick S.’ as is usual practice.

Robin said there were no grounds to suspect that Lubitz was carrying out a terrorist attack. ‘Suicide’ was also the wrong word to describe actions which killed so many other people, the prosecutor added: “I don’t necessarily call it suicide when you have responsibility for 100 or so lives.”

The family of the co-pilot arrived in France for a tribute alongside other victims but was being kept apart from the others, Robin said. Police set up guard outside Lubitz’s house in Montabaur, Germany.

An employee of the German Federal Bureau of Aircraft Accident Investigation holding a voice recorder, an up-to-date model by L-3 Aviation Products that is used in Boeing and Airbus aircraft.

Robin said the conversation between the two pilots before the captain left the cockpit started normally but that Lubitz’s replies became “laconic” as they started readying what would have been the normal descent to the airport of Düsseldorf.

“His responses become very brief. There is no proper exchange as such,” he said.

Investigators were still searching for the second of the two black boxes yesterday in the ravine where the plane crashed, 100 km from Nice, which would contain data from the plane’s instruments.

Pilots may temporarily leave the cockpit at certain times and in certain circumstances, such as while the aircraft is cruising, according to German aviation law.

Cockpit doors can be opened from the outside with a code, in line with regulations introduced after the September 11, 2001 attacks in the US, but the code can be overridden from inside the cockpit. Lufthansa’s CEO said that either the pilot had entered the code incorrectly, or the co-pilot inside had overridden it.