Marseille prosecutor Brice Robin has confirmed the co-pilot of Germanwings A320 flight 4U9525 was alone inside the cockpit when the plane crashed, killing 150 people, and he appeared to want to "destroy the plane".

The co-pilot, named as Andreas Lubitz, a 28-year-old German national from Rhineland, was alive when the plane hit the ground, as revealed by a "breathing noise" from the recording. His breathing was normal throughout the final minutes of the flight, hinting he was conscious and deliberate in his actions.

"The co-pilot manipulated the flight monitoring system to voluntarily start descent of the plane. He did it in a deliberate way," the prosecutor said. "We could hear appeals from the pilot to access the cockpit, but no response from the cockpit."

The prosecutor said Lubitz "had no reason to do it" and had no links to any terrorist groups. "Nothing to suggest this was a terrorist act," Robin said. He added the passengers' death was instant, hitting the mountain at 700km/h.

"I don't think the passengers realised what was happening until last moments because on recording you only hear screams in final seconds." Robin said.

Earlier, the New York Times quoted a senior military official as saying one of the two pilots left the cockpit and could not get back in.

He said: "The guy outside is knocking lightly on the door, and there is no answer. And then he hits the door stronger, and no answer. There is never an answer. You can hear he is trying to smash the door down."

The investigator, who has not been named, said many questions remain unanswered on the reasons why one of the pilots went out. "But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door," he said.

The captain, who has been confirmed as the one locked out when the plane crashed, had years of experience and more than 6,000 hours of flying time. He had been a Germanwings pilot since May 2014. Meanwhile, Lubitz had just 630 flying hours and joined Germanwings straight after training in September 2013.

The Airbus A320, operated by Germanwings, was travelling to Dusseldorf from Barcelona when it descended into the French Alps, killing the 144 passengers and six crew members on board.

French authorities had a full transcript of the final 30 minutes of the voice recorder. "During the first 20 minutes, the pilots exchange communication in a normal, amicable way," Robin said. "Then we hear the pilot preparing the landing briefing to Dusseldorf. The co-pilot answer was laconic."

"Then, we could hear the pilot asking the co-pilot to take control. We hear the noise of seat speed move, door closed and shut. He left the cockpit to go to the toilet," the prosecutor said.

"At this stage, the co-pilot is in control, alone," the prosecutor said. "It is when he is alone that the co-pilot manipulates the flight monitoring system to activate the decent of the plane." The prosecutor said that this action could only have been "deliberate."