French prosecutors said on Thursday that a Germanwings co-pilot appeared to want to "destroy the plane," according to information pulled from the black box cockpit voice recorder.

The co-pilot did not say a word once the captain left the cockpit. "It was absolute silence in the cockpit," said Marseille prosecutor Brice Robin at a Thursday press conference.

He said that pounding could be heard on the cockpit door during the final minutes as alarms sounded. The co-pilot "voluntarily" refused to open the door and was alone at the controls of the Germanwings flight that slammed into an Alpine mountainside and "intentionally" sent the plane into the doomed descent.

Robin said the co-pilot's breathing was normal throughout the final minutes of the flight. He identified the pilot as a German national who had never been flagged as a terrorist.

Earlier on Thursday, an official with knowledge of the audio recordings said one of the pilots apparently was locked out of the cockpit when the plane went down.

The official, who was not authorized to speak publicly about the investigation, said that the details emerged from recordings recovered from the black box found on Wednesday among the debris of the aircraft.

The New York Times earlier quoted an unidentified investigator as saying someone could be heard knocking on the cockpit door.

One of the pilots is heard leaving the cockpit, then banging on the door with increasing urgency in an unsuccessful attempt to get back in.

"The guy outside is knocking lightly on the door and there is no answer," the paper quoted an unidentified investigator as saying. "And then he hits the door stronger and no answer. There is never an answer."

Eventually, the newspaper quotes the investigator as saying, "You can hear he is trying to smash the door down."

"We don’t know yet the reason why one of the guys went out,” said the investigator quoted in the Times. "But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door."

Lufthansa chief executive Carsten Spohr described the pilots as "experienced and trained" in a press conference in Barcelona, Spain, on Wednesday night.

Lufthansa has refused to identify the pilots or give details of ages and nationality, but it said the co-pilot joined Greenwings in September 2013 directly after training and had flown 630 hours.

The captain had more than 6,000 hours of flying time and been Germanwings pilot since May 2014, having previously flown for Lufthansa and Condor, Lufthansa said.

The retrieval of the black box came as French President François Hollande, Germany's Angela Merkel and Spain's Mariano Rajoy traveled to the crash site in a remote French Alpine region to pay tribute to the 150 victims, most of them Germans and Spaniards.

While Hollande promised that authorities would not rest until the causes of the crash were known, France's BEA air incident investigation bureau said it was still far too early to draw meaningful conclusions on why the plane, operated by the Germanwings budget arm of Lufthansa, went down over the southern French Alps during a routine flight from Barcelona to Düsseldorf, Germany.

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

German Interior Minister Thomas de Maiziere told reporters in Berlin on Wednesday that "according to the latest information, there is no hard evidence that the crash was intentionally brought about by third parties." He said that authorities are nevertheless investigating all possible causes for the crash.

Helicopters surveying the scattered debris lifted off at daybreak Wednesday, and crews traveled slowly overland to the remote crash site through fresh snow and rain, threading their way to the craggy ravine.

Key to the investigation is what happened at 10:30 and 10:31 a.m., said Ségolène Royal, a top government minister whose portfolio includes transport. After that, air traffic controllers were unable to make contact with the plane.

The voice recorder takes audio feeds from four microphones in the cockpit and records all the conversations between the pilots and air traffic controllers as well as any noises in the cockpit. The flight data recorder, which French Interior Minister Bernard Cazeneuve said has not been retrieved yet, captures 25 hours' worth of information on the position and condition of almost every major part in a plane.

Royal and Cazeneuve emphasized that terrorism is considered unlikely in the crash.

The crash scattered shards of debris across several acres and left pieces of wreckage "so small and shiny, they appear like patches of snow on the mountainside," said Pierre-Henry Brandet, the Interior Ministry spokesman, after flying over the debris field.

Investigators retrieving data from the recorder will focus first "on the human voices, the conversations" and then on cockpit sounds, Transport Secretary Alain Vidalies told Europe 1 radio. He said the government planned to release information gleaned from the black box as soon as it can be verified.