Germanwings pilot 'locked out of cockpit' before French Alps crash
Debris from the Germanwings Airbus A320 is seen strewn over the crash site in the French Alps above the southeastern town of Seyne-les-Alps (AFP)

One of the two pilots on the Germanwings flight that crashed in the French Alps was locked out of the cockpit before the plane went down, killing 150 people, a source close to the investigation told AFP Thursday.

The breakthrough in the investigation, which only increased speculation as to the cause of the disaster, came as bereaved families began arriving from Spain and Germany near the remote mountainous crash site.

Cockpit recordings recovered from the crash site indicated one of the seats was pushed back and the door opened and closed. Then knocking is heard, said the source, adding "there was no more conversation from that point until the crash".

The source said an alarm indicating the proximity to the ground could be heard before the moment of impact.

All 150 people on board, including two babies and 16 German school exchange pupils, died when the Germanwings flight 4U 9525 from Barcelona to Duesseldorf smashed into the mountains after an eight-minute descent.

The Airbus A320 sent no distress signal and the crew failed to respond to ground control's desperate attempts to make contact.

The cockpit recording showed the pilots speaking normally in German at the start of the flight, the source said, adding that it could not be determined if it was the captain or the first officer who left the cockpit.

The New York Times cited a senior military official involved in the investigation as saying the black box data indicated one pilot tried unsuccessfully to bash his way back in to the cockpit.

"The guy outside is knocking lightly on the door, and there is no answer," the investigator told the newspaper. "And then he hits the door stronger, and no answer. There is never an answer."

He continued: "You can hear he is trying to smash the door down."

- 'Unconscious or dead' -

In a statement overnight, Germanwings said that "at the moment, we do not have information from competent authorities to confirm this story.""We are doing everything to get the most information possible and we are not engaging in speculation."

Germanwings' parent company Lufthansa said the co-pilot had been working for them since September 2013 and had 630 hours of flight experience.

The pilot had more than 10 years experience and 6,000 hours flying time.

Under strengthened security measures introduced after the September 11, 2001 hijackings and attacks, authorisation to open a cockpit door can only come from inside and from a pilot.

Authorities have said they have no explanation as yet for the tragedy, but said that the plane was still flying when it crashed into the mountain and did not explode mid-air.

Aviation experts interviewed by AFP suggested several theories. "If the pilots did not stop the airplane from flying into the mountains, it is because they were unconscious or dead, or they had decided to die, or they were forced to die," one expert told AFP.

The prosecutor in the southern city of Marseille, who is leading the judicial enquiry into the accident, was due to brief reporters at 12:30 pm local time (1130 GMT).

On Wednesday, the head of France's BEA crash investigation agency head Remi Jouty told reporters he still had "not the slightest explanation" for the tragedy at this stage.

"It is inexplicable," Lufthansa chief executive Carsten Spohr said in Frankfurt.

"The plane was in perfect condition and the two pilots were experienced."

Investigators are still hunting for the second black box, which would reveal technical flight data.

President Francois Hollande said on Wednesday the casing of this box had been found but not the device itself.

France's interior minister has said the probe is not focusing on a possible terror attack.

- Families gather -

Meanwhile, families were heading for a hamlet close to the crash site to say a final farewell to their loved ones, at least 51 of whom were Spaniards and at least 72 Germans.

Helicopters began winching the remains of victims, found scattered across the scree-covered slopes, to nearby Seyne-les-Alpes Wednesday, a source close to the investigation told AFP.

Authorities on the ground who had the gruesome task of sifting through the debris resumed the search at dawn Thursday.

A mountain guide who got near the crash site said he was unable to make out recognisable body parts.

"It's incredible. An Airbus is enormous. When you arrive and there's nothing there... it's very shocking," said the guide, who did not wish to be identified.

The crash site, which is situated at about 1,500 metres (5,000 feet) altitude, is accessible only by helicopter or an arduous hike on foot.

French President Francois Hollande and German Chancellor Angela Merkel flew over the site to see the devastation for themselves Wednesday. Spain's Prime Minister Mariano Rajoy also visited a crisis centre near the scene.

The plane was carrying six crew and 144 passengers, including 16 German teenagers returning home from a school trip, who had reportedly won the trip in a lottery of their classmates.

It was the deadliest air crash on the French mainland since 1974 when a Turkish Airlines plane crashed, killing 346 people.

Lufthansa said the aircraft was carrying citizens of 18 countries. Three Americans and three Britons were confirmed among the victims.

Argentina, Australia, Belgium, Colombia, Denmark, Holland, Israel, Japan, Mexico and Morocco also had nationals on board, according to officials.