Marignane (France) (AFP) - The flight started like any other, the conversation in the cockpit perfectly normal, with co-pilot Andreas Lubitz offering no indication of the horror he would soon allegedly inflict on the other 149 people on board flight 4U 9525.

"For the first 20 minutes, they spoke in a normal fashion, courteous, like normal pilots. There was nothing abnormal," French prosecutor Brice Robin told reporters on Thursday.

He was relaying information from the cockpit voice recorder, captured by one of the "black boxes" discovered among the debris of the Airbus A320 that crashed into the French Alps on Tuesday.

The plane had taken off from Barcelona around 10am local time, heading for Duesseldorf in Germany.

When the plane reached cruising altitude and been placed on auto-pilot, Lubitz and the other pilot, who has yet to be named, began discussing the landing in Duesseldorf.

The responses from Lubitz remained normal, but "very short... not a real dialogue," Robin said.

"Then we hear the pilot ask the co-pilot to take the controls and we hear the sound of a seat being pulled back and a door closing. We can assume he left to answer nature's call."

Once left alone, Lubitz turned a button on the flight monitoring system that began the plane's descent.

"This action can only be deliberate," Robin said. "It would be impossible to turn the button by mistake. If you passed out and leaned over on it, it would only go a quarter-way and do nothing."

The pilot returned from the toilet and tried to open the cockpit door, which is heavily reinforced to prevent hijackings and requires a code.

He may not have known the code, although Lufthansa chief Carsten Spohr said this was unlikely.

More probable, officials said, is that Lubitz deliberately locked the door from the inside, making it impossible for anyone to enter.

"The most plausible interpretion is that the co-pilot deliberately refused to open the cockpit door to the pilot," Roin said.

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The black box then records increasingly frantic attempts by the pilot to break down the door, to which Lubitz makes no response.





- 'Total silence' -





For the eight minutes after he began the descent, Lubitz was apparently calm and silent, breathing normally and showing no sign of panic.

"He does not say a single word. Total silence," Robin said.

The descent started over the town of Toulons in southern France, triggering concern from air traffic controllers who repeatedly tried to contact the plane.

As alerts came in on the fall in altitude and other changes, there was no response and no distress message from the plane.

Desperate ground controllers even asked nearby planes to try to make contact, but they, too, were unsuccessful.

The plane dropped gradually from around 10-12,000 metres (30,000 feet) to 2,000 m -- slowly enough, Robin said, that passengers would have been unaware anything was wrong.

"I think the victims were only aware at the very last moment. The screams are heard only in the last instants before the impact," said Robin.

The first sign of trouble would have come when alarms were automatically set off to indicate the plane was dangerously close to the ground.

Within moments, the plane had slammed into a mountainside at a speed of 700 kilometres per hour, Robin added.

Lubitz's motives remain entirely unknown.

Nor do the investigators know what would have happened if the pilot had not gone to the toilet.

"I'm not in the head of this co-pilot. Normally when we commit suicide, we do it alone. When you're responsible for 150 people behind, I don't necessarily call that suicide," Robin said.