However, they are currently not calling the crash a terrorist act, nor a suicide.

The co-pilot, identified as Andreas Lubitz, repeatedly turned a dial to override the plane's autopilot, sending it in a steep descent into the Alps, after the plane's captain had left the cockpit to go to the toilet.

The last eight minutes of the recovered cockpit flight recorder revealed the man's steady, measured breathing as he ignored queries from flight control, an altitude alarm from the plane's dashboard, and finally the desperate shouts of the captain outside.

At the very end of the recording, the screams of passengers can be heard, just before the plane hits the mountain.

"After the voice recorder was analysed of our tragic flight, Germanwings flight 9525, we had quite a turn and information,"  said Lufthansa Airlines CEO Carsten Spohr in a press conference. "We from Lufthansa are speechless that this aircraft has been deliberately crashed by the co-pilot.

"Recordings from the voice recorder showed the captain has left the cockpit just for a short time in the beginning and that he could not go back," he said. "It seems to be true that the colleague, who was still in the cockpit, the co-pilot did not give the captain access again in order to activate the fatal descent into the Alps. This makes us speechless, we here at Lufthansa, Germanwings.

"We are shocked and in dismay and I actually could not have imagined that this is even getting worse."

In an earlier press conference in Marseille, Public Prosecutor Brice Robin described what investigators had heard on the cockpit voice recorder, a bright orange box found on Tuesday amid the debris scattered down a steep, dark ravine near Seyne-les-Alpes.

Early in the flight there was a "lively and courteous" conversation between the pilots, talking to each other in "a very normal way".

The captain then began preparing his briefing for landing in Dusseldorf.

The co-pilot was "rather laconic" in his replies, Mr Robin said. "You would have expected more of a dialogue, not these curt answers."

The captain then asked the co-pilot to take over the plane from him – apparently so he could go to the toilet.

There was the sound of the captain's seat going back, then the cockpit door closed behind him, locking automatically and leaving only the co-pilot in the cabin.

Almost immediately, the co-pilot adjusted a control button that overrode the autopilot and set the plane into a descent.

Such an action "can only be voluntary", Mr Robin said. "It can only be deliberate."

It required several turns, not just one, in order to lower the altitude enough to cause the crash. "This can't be done automatically."

The next sound was the captain asking to be let back into the cockpit, over the intercom.

After 9/11, international regulations require that someone inside the cockpit must manually unlock the armoured cockpit door, on a request from outside.

The co-pilot did not do so, and he did not answer the captain.

From then, from the copilot, "you can only hear normal human breathing" until the final crash. It did not sound like the breathing of someone who was unwell or unconscious, or someone in a state of panic.

Air traffic control in Marseille tried to get in touch with the plane several times, and their requests could be heard coming over the radio.

They then sent a special request asking for an emergency transponder signal to give the plane priority over other planes, assuming it was in distress.

They also asked other planes to try and contact the flight – but there was no answer. No Mayday was sent from the plane.

Alarms went off, signaling that the ground was close.

Then there was the sound of "violent knocks" of the captain trying to get the door open.

Then "you can hear people screaming at the end, just before impact".

Finally, there is the noise of the crash. "Death was instantaneous," Mr Robin said. "At that speed, slamming into the mountainside, the plane was pulverised."

The most likely interpretation, Mr Robin said, is that the co-pilot deliberately did not answer the captain's request to come in, and deliberately changed the course of the plane.

"It can be analysed as a willingness to destroy the aircraft," he said. "We are not using the word suicide … At this moment what I can say is he voluntarily allowed the plane to descend, to lose altitude at 1000m per minute. It's not normal, he did not have any reason to do this.

"Usually you commit suicide by yourself. When you are responsible for 150 people behind you, I don't know if you would use the word 'suicide'. But it is a legitimate question to ask, indeed."

The team are now treating it as intentional manslaughter, he said.

One hundred and fifty people died in the crash, including a mother and son from Victoria, and a group of schoolchildren from Germany.

The French investigators have contacted German authorities to find more information about the co-pilot. Andreas Lubitz was a German national, 28 years old, but the prosecutor did not know his ethnic origin. He had around 630 hours of experience on this type of aircraft.

"He is not considered to be a terrorist, he is not on a terrorist list," Mr Robin said.

On Thursday search and rescue teams continued to recover human remains from the crash site, near Seyne-les-Alpes.

The long process of identifying the remains by matching it to DNA from relatives has begun.

The crash investigators are also still trying to locate the second black box, which contains all the data readouts from the plane's systems.

"This will make it possible to really know there was no other reason for the crash," Mr Robin said.

The search and rescue teams are working to recover human remains from the two-hectare crash site, between 1500m and 2000m in altitude on the mountainside, which is "rugged, steep and unsafe", Mr Robin said.

"Stones are constantly rolling down the mountain – it's a very complicated crash site."

The remains recovered from the site are put in bags then taken by helicopter to a nearby location where the forensic identification process begins.

Two plane-loads – around 200 - of the victims' relatives have arrived in the region, and the families have been told the latest news.

"They are in a state of shock," Mr Robin said. "They have had difficulty coming to terms with the reality – they have asked a lot of questions."

They are being taken to a chapel near Seyne that has been set up for them to lay flowers and pray for their lost relatives.

They will also provide DNA to help with the identification process, however they will not be able to leave with their loved ones' remains until the whole process is finished, which could take weeks.

- With Daniel Fallon

smh.com.au