General view of the interior of the cockpit of an A320 aircraft belonging to Jetstar Asia airways, at Singapore airport January 31, 2015.[Photo/Agencies]

SEYNE-LES-ALPES/PARIS - Investigators have retrieved cockpit voice recordings from one of the black boxes of the German Airbus plane that smashed into the Alps, killing everyone onboard, and they expect a preliminary read-out of their content in days, an official said on Wednesday.

The development came as French President Francois Hollande, Germany's Angela Merkel and Spain's Mariano Rajoy travelled to the crash site in a remote French Alpine region to pay tribute to the 150 victims, mostly Germany and Spanish.

"We have just been able to extract a useable audio data file," BEA director Remi Jouty told a news conference at its headquarters outside Paris.

One of the pilots on the German Airbus plane that crashed in the French Alps, killing everyone onboard, left the cockpit and was unable to return before the plane went down, the New York Times reported on Wednesday, citing evidence from a cockpit voice recorder.

"The guy outside is knocking lightly on the door and there is no answer," an unnamed investigator told the Times. "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.