The co-pilot of a plane which crashed in the Alps activated the descent button and refused to open the cockpit door to the pilot.

Marseille prosecutor Brice Robin says the co-pilot, Andreas Lubitz (28), was alone at the controls of the Germanwings flight and "intentionally" sent the plane into the doomed descent.

He said that the crew member wanted to "destroy the plane".

He said: "We assume the (captain) went to the loo or something. The co-pilot is on his own in charge of the plane, and it is while he is alone that he uses the flight monitoring system which starts the descent of the plane."

The flight monitoring system cannot be accidentally triggered, he added.

The plane ploughed into the side of a mountain at around 430mph, killing all of those on board instantly.

"I think the victims only realised at the last moment because on the recording you only hear the screams literally on the last moments of the recording."

Mr Robin said Mr Lubitz was a German national - but does not know his ethnicity or religion.

He said there is nothing to indicate that this was a terrorism-related event. He said he would not speculate on whether the co-pilot had committed suicide.

Mr Lubitz is understood to have joined the airline in 2013 straight after training.

Breathing could be heard from the cockpit and was normal, which has led investigators to believe he was conscious at the time.

Some 500 people are working on the investigation, which is hampered by the remote location of the crash.

Each body must be removed by helicopter as the mountainside is very steep.

Mr Robin says the co-pilot appears to have crashed deliberately.

It comes after earlier reports that one of the pilots was locked out of the cockpit before it started its descent, according to US media.

The New York Times quoted a senior military source involved in the probe as saying one of the two Germanwings pilots appeared to have left the cockpit and then been unable to get back in.

The source said: "The guy outside is knocking lightly on the door and there is no answer. 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 official added: "We don't know yet the reason why one of the guys went out. But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door."

The source said conversation between the pilots during the early part of the flight from Barcelona to Dusseldorf had been "very smooth, very cool."

The report emerged after a bus carrying the first relatives to the area where their loved ones lost their lives left Barcelona on Wednesday evening.

The group of 14 is meeting with other family members who chose to fly from Barcelona to Marseille this morning.

Meanwhile a book of condolence for victims of the crash has opened in Dublin.

The Lord Mayor of Dublin, Christy Burke, was joined by the German and Spanish Ambassadors to Ireland - as well as the the Chargé d'Affaires of the French Embassy in Ireland - at the Mansion House.

Members of the public will be able to offer their condolences between 11pm and 4pm today. It will remain open until Saturday.

Lord Mayor Burke says this is an opportunity for people pay their respects to those who lost their lives.

Earlier, Germanwings parent company Lufthansa said the plane, piloted by two experienced captains, was "technically flawless", while the firm's CEO Carsten Spohr, himself a pilot, described the crash as "inexplicable".

Lufthansa said: "We cannot comprehend how a technically flawless airplane steered by two experienced pilots could encounter such a situation at cruising altitude."

"All of us at Lufthansa are working to ensure that such an incident will never occur again. We cannot believe that this has happened. We are doing everything to support the families."

A spokesman for the French air investigation bureau, the BEA, said that "usable" material had been extracted from the black box cockpit voice recorder found at the crash site.

He said it covered the entire flight but would not say what conversations, if any, between the pilots had been captured on the recording, nor what language they had been conducted in.

Three Britons were among 150 on the Airbus A320 plane to have lost their lives.

While some nationalities were still to be verified, Germanwings said the dead included 72 Germans, 35 Spaniards and two Australians.

Two Americans are also known to have been on the plane.

Other countries with passengers on the flight included Iran, Israel, Japan, Denmark, Belgium and Colombia.