The co-pilot of a Germanwings flight that slammed into an Alpine mountainside "intentionally" sent the plane into its doomed descent, killing all 150 people on board, a French prosecutor has said.

Marseille prosecutor Brice Robin said on Thursday that the commander left the cockpit, presumably to go to the lavatory, and then was unable to regain access.

In the meantime, he said, co-pilot Andreas Lubitz manually and "intentionally" set the plane on the descent that drove it into the mountainside in the southern French Alps.

It was the co-pilot's "intention to destroy this plane," Robin said.

The information was pulled from the black box cockpit voice recorder, but Robin said the co-pilot did not say a word after the commanding pilot left the cockpit.

'Alive until impact'

"We hear the captain then speaks via an interphone to the co-pilot, no response of co-pilot, he taps on door, no response of co-pilot, all we can hear is the sound of breathing, until impact suggesting the co-pilot was alive until impact," Robin said.

The Airbus A320, on a flight from Barcelona to Dusseldorf, began to descend from cruising altitude after losing radio contact with ground control and slammed into the remote mountain on Tuesday morning.

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German Interior Minister Thomas de Maiziere said that current information suggested that the co-pilot had no links to terrorism.

"According to the current state of knowledge and after comparing information that we have, he does not have a terrorist background," he said.

In the German town of Montabaur, acquaintances said Lubitz was in his late twenties and showed no signs of depression when they saw him last fall as he renewed his glider pilot's license.

"He was happy he had the job with Germanwings and he was doing well," said a member of the glider club, Peter Ruecker, who watched him learn to fly. "He gave off a good feeling."

Lubitz had obtained his glider pilot's license as a teenager, and was accepted as a Lufthansa pilot trainee after finishing a tough German college preparatory school, Ruecker said. He described Lubitz as a "rather quiet" but friendly young man. Lufthansa has yet to officially identify the pilots but said the co-pilot joined Germanwings in September 2013, directly after training, and had flown 630 hours. The head of Lufthansa, Germanwings' parent company, said no security "in the world" could protect an airline from the kind of action taken by the co-pilot.

"Whatever safety provisions you have in a company, however high the standards, such an isolated case cannot be completely ruled out," Lufthansa chief executive Carsten Spohr told reporters.

Families brought to France

The families of victims were being flown to Marseille on Thursday before being taken up to the zone close to the crash site.

Al Jazeera's Charlie Angela, reporting from near the crash site in the French Alps, said that two planes carrying the families were bound for Marseille and both groups were to travel on by road to the site.

TIMELINE: Major air disasters Our correspondent added that planes took off from Barcelona and Dusseldorf, adding that some relatives who preferred not to fly were travelling by bus from the Spanish city.

"The aim is to get these families as close as possible to the crash site. It is two hours on foot. Some of them might also be flown over the crash site."