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.

Andreas Lubitz

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.

Prosecutors in the German city of Duesseldorf 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.

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."

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." in accordance with usual practice.

"SUICIDE" THE WRONG WORD

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. Acquaintances in the town said they were stunned.

"I'm just speechless. I don't have any explanation for this. Knowing Andreas, this is just inconceivable for me," said Peter Ruecker, a long-time member of the local flight club where Lubitz received his flying license years ago.

"He was a lot of fun, even though he was perhaps sometimes a bit quiet. He was just another boy like so many others here."

A photo on Lubitz's Facebook page, which was later taken down, shows a smiling young man posing in front of San Francisco's Golden Gate bridge.

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 Duesseldorf.

"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 on Thursday 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 United States, 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.

Germanwings said 72 Germans were killed in the first major air passenger disaster on French soil since the 2000 Concorde accident just outside Paris. Madrid revised down on Thursday the number of Spanish victims to 50 from 51 previously.

As well as Germans and Spaniards, victims included three Americans, a Moroccan and citizens of Britain, Argentina, Australia, Belgium, Colombia, Denmark, Israel, Japan, Mexico, Iran and the Netherlands, officials said. However, DNA checks to identify them could take weeks, the French government said.

The families of victims were being flown to Marseille on Thursday before being taken up to the zone close to the crash site. Chapels had been prepared for them with a view of the mountain where their loved ones died.