Voice recordings from the German jet that crashed in the Alps showed one of the pilots left the cockpit and could not get back in before the plane went down, killing all 150 onboard, the New York Times reported.
The recordings did not make clear why the pilot left the cockpit or why he could not regain entry as the plane operated by budget airline Germanwings steadily descended toward a mountain range in a remote area of the French Alps on Tuesday.
Investigators were studying the voice recordings from one of the "black boxes" for answers on Thursday while the search continued for a second in the ravine where the plane crashed, 100 km (65 miles) from the Riviera city of Nice.
"The guy outside is knocking lightly on the door and there is no answer," an investigator described only as a senior French military official told the New York Times, citing the recordings. "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 added.
The cockpit audio had showed "very smooth, very cool" conversation between the pilots in the early part of the flight.
"We don't know yet the reason why one of the guys went out," the official said. "But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door."
A spokesman for Germanwings' owner Lufthansa said: "We have no information from the authorities that confirms this report and we are seeking more information. We will not take part in speculation on the causes of the crash."
It confirmed that the main pilot had over 6,000 hours of flying time, while the more junior co-pilot had just 630 hours and had been with Germanwings since September 2013.
In France, the interior and defense ministries said they had no information on the newspaper report. The prosecutor of the city of Marseille handling the legal side of the investigation was due to give a news conference at 1130 GMT, and Lufthansa also announced a briefing for later in the day.
France's BEA air investigation bureau was not available for comment on the report. On Wednesday, it said it was too early to draw meaningful conclusions on why the plane went down.
"We have not yet been able to study and to establish an exact timing for all the sounds and words heard on this file," BEA director Remi Jouty told a news conference.
Jouty expected the first basic analysis in days but warned that the read-out could be subject to errors and that more work would be needed for a full interpretation.
Although he said "words" had been heard on the tape, Jouty would not confirm whether that meant the Airbus A320's pilots were conscious, and he gave no details of the recordings.
POST-9/11 RULES
The BEA said the plane started descending a minute after reaching cruising height and lost altitude for over nine minutes. The pilot's last words to the ground confirmed the next navigational waypoint, ending with a call-sign and "thank you".
Pilots may temporarily leave the cockpit at certain times and in certain circumstances, such whilst the aircraft is cruising, according to German aviation law.
Lufthansa said that its cockpit doors can be opened from the outside with a code, in line with regulations introduced after the Sept. 11 attacks. However, the code system can be blocked from inside the cockpit, according to an Airbus promotional video posted online and confirmed by the planemaker.
The BEA on Wednesday already ruled out a mid-air explosion and said the scenario did not look like a depressurisation.
It also noted the airliner had flown in a straight line directly into the mountain, but had no word on whether that seemed to be at the hand of a pilot or auto-pilot.
The retrieval of one of two black boxes came on Wednesday as French President Francois Hollande, Germany's Angela Merkel and Spain's Mariano Rajoy visited the crash site to pay tribute to the victims, mostly Germany and Spanish.
Germanwings said 72 Germans were killed in the first major air passenger disaster on French soil since the 2000 Concorde accident just outside Paris. Madrid revised down on Thursday the number of Spanish victims to 50 from 51 previously.
As well as Germans and Spaniards, victims included three Americans, a Moroccan and citizens of Britain, Argentina, Australia, Belgium, Colombia, Denmark, Israel, Japan,Mexico, Iran and the Netherlands, officials said. However, DNA checks to identify them could take weeks, the French government said.
The families of victims were being flown to Marseille on Thursday before being taken up to the zone close to the crash site. Chapels had been prepared for them with a view of the mountain where their loved ones died.
Police and forensic teams on foot and in helicopters pursued searches but said the impact of the crash was so violent that the airplane had been shattered into small pieces.
"When we go to a crash site we expect to find part of the fuselage. But here we see nothing at all," said Xavier Roy, coordinating air operations.
