One of the two pilots on the Germanwings flight was locked out of the cockpit before the plane crashed in the French Alps, killing 150 people, a source close to the investigation has told the AFP agency.
The New York Times also cited a senior military official involved in the investigation as saying the black box data indicated one pilot was locked out of the cockpit and tried unsuccessfully to bash his way back in.
"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."
Cockpit recordings recovered from the site of Tuesday's crash indicated one of the seats was pushed back and the door opened and closed.
The AFP source said on Thursday that knocking could be heard, 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 those on board were killed when the Germanwings flight 4U 9525 from Barcelona to Duesseldorf crashed in remote terrain, after an unexplained eight-minute descent in mid-flight.
No distress signal was sent from the Airbus A320 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 AFP source said, adding that it could not be determined if it was the captain or the first officer who left the cockpit.
"And then he hits the door stronger, and no answer. There is never an answer
Photos of the mangled black box retrieved at the site showed its metal casing torn and twisted by the violence of the impact.
The casing of a second black box, the flight data recorder, has been found but not the device itself.
Investigators have said the plane was still flying when it smashed into the remote mountainside, with the force of the impact leaving only small pieces of debris scattered over a wide area.
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 altitude, is accessible only by helicopter or a hike on foot.
On Wednesday, Remi Jouty, the head of France's Bureau d'Enquetes et d'Analyses (crash investigation agency) told reporters he still had "not the slightest explanation" for the tragedy at this stage.
Speaking in Frankfurt, Lufthansa chief executive Carsten Spohr said: "It is inexplicable. The plane was in perfect condition and the two pilots were experienced."
Weather did not appear to be a factor in the crash, with conditions calm at the time, and Germanwings had an unblemished safety record.
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.