Seyne-les-Alpes (France) (AFP) - One of the pilots on the doomed Germanwings flight was locked out of the cockpit shortly before the plane crashed in the French Alps, killing all 150 aboard, a source told AFP, raising new questions for investigators trying to make sense of the tragedy.
The news came as families and friends of victims began arriving in France to travel to the remote mountainous crash site area, where locals have opened their doors in a show of solidarity with the grieving relatives.
Cockpit recordings from one of the plane's black boxes indicated that a seat was pushed back, then the door opened and closed. Later, knocking is heard, said the source close to the probe, adding "there was no more conversation from that point until the crash".
The source, who asked not to be identified, said an alarm indicating proximity to the ground could be heard before the moment of impact.
All 150 people on board flight 4U 9525, including two babies and 16 German school exchange pupils, died when the Airbus A320 from Barcelona to Duesseldorf smashed into the mountains after an eight-minute descent.
There was no distress signal from the plane 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. A second black box, which records flight data, has not yet been recovered.
The New York Times cited a senior military official involved in the investigation as saying the cockpit black box recording 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' -
Germanwings told AFP: "Only a pilot inside the cockpit can unlock the door."
But its spokesperson refused to either confirm or deny "for security reasons" whether there was any way to open the door from outside, perhaps with an access code.
They confirmed the existence of a video surveillance system that allows the pilot to see who is trying to enter the cockpit.
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.
However, neither pilot has been identified yet.
Authorities say the plane was flying right until the moment of impact and that there was no mid-air explosion.
The French interior minister has said that terrorism is not considered likely. However, aviation experts say the mystery remains wide open.
"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.
Earlier, Lufthansa chief executive Carsten Spohr called the incident "inexplicable".
"The plane was in perfect condition and the two pilots were experienced," he said.
The prosecutor for the southern city of Marseille, who is leading the judicial enquiry, was due to brief reporters at 12:30 pm (1130 GMT).
- Grieving families gather -
Meanwhile, two planes arrived in southern France on Thursday from Barcelona and Duesseldorf with families and friends of victims.
They were due to meet the prosecutor before heading by bus to the hamlet close to the crash site.
Tents were set up for them to give DNA samples to start the process of identifying the bodies of loved ones, at least 51 of whom were Spaniards and at least 72 Germans.
The remains of victims, found scattered across the scree-covered slopes, were being taken by helicopter to nearby Seyne-les-Alpes, a source close to the investigation told AFP.
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.
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.