Police in the German city of Düsseldorf have searched both homes of 28-year-old Andreas Lubitz, who is believed to have deliberately crashed Germanwings flight 4U 9525 into the French Alps on Tuesday.

"Both the home of the co-pilot in Düsseldorf and the home in Montabaur have been searched," said chief public prosecutor Ralf Herrenbrueck.

Mr Lubitz "voluntarily" initiated the descent of the Germanwings flight that crashed into the French Alps and refused to open the door to the pilot who was outside the cockpit, the lead investigator in France said.

Marseille prosecutor Brice Robin said Mr Lubitz seemed to "show a desire to want to destroy the plane".

"The co-pilot is alone at the controls," said Mr Robin, recounting information gathered from the "black box" recorder.

"He voluntarily refused to open the door of the cockpit to the pilot and voluntarily began the descent of the plane."

Mr Robin told a press conference he had received the information late last night.

He said: "The co-pilot uses the flight monitoring system to start the descent of the plane. This can only be done voluntarily, not automatically.

"We hear several cries from the captain asking to get in. Through the intercom system he identifies himself - but there is no answer.

"He knocks on the door and asks for it to be opened - but there is no answer."

Asked whether he believed the crash that killed 150 people was the result of suicide, Mr Robin said: "People who commit suicide usually do so alone ... I don't call it a suicide."

He also said the cockpit voice recording suggests that passengers' screams began just before the final impact.

The Airbus A320 hit the mountainside at 700km/h and the passengers died "instantly", he added.

Flight 4U 9525, which was en route from Barcelona to Düsseldorf, crashed in the French Alps on Tuesday morning.

Mr Robin told a press conference in Marseille the German co-pilot was not known as a "wanted terrorist".

He said the sounds on the "black box" suggest Mr Lubitz was alive and breathing until impact and "was breathing normally, it wasn't the breathing of someone who was struggling".

Lufthansa chief executive Carsten Spohr said he was "stunned" by suggestions the co-pilot of the jet operated by its Germanwings subsidiary had deliberately crashed the plane.

Mr Spohr told a news conference there was "no indication what might have led" to the actions of Mr Lubitz.

He said Mr Lubitz had passed all psychological tests required to begin training and underwent regular physical examinations.

Lufthansa said Mr Lubitz had been on the job since September 2013 and had 630 hours of flying experience.

Before that he had been trained as a pilot at Lufthansa Flight Training in the German city of Bremen.

The pilot had more than ten years experience and 6,000 hours of flying the Airbus A320 model.

No distress signal was sent from Flight 4U 9525 and the crew failed to respond to ground control's desperate attempts to make contact.

No grounds to consider crash as 'terrorist incident'

Mr Robin said there are currently no grounds to consider the crash as a "terrorist incident".

He added: "I believe that we owe the families the transparency of what the investigation is pointing to and what is going on, we owe it to them to tell them what happened.

"The families have been informed of everything I just told you."

The prosecutor said the cockpit voice recorder gave information from the first 30 minutes of the flight.

For the first 20 minutes the two pilots talked in a normal fashion and were as courteous as two normal pilots would be.

Then the captain is heard asking the co-pilot to take over and the sound of a chair being pushed back and a door being closed is heard.

"Not in our worst nightmare could we imagine something like this happening," said Germanwings following the description of the plane's last moments.

Earlier, the New York Times reported that the voice recordings showed one of the pilots left the cockpit and could not get back in before the plane went down.

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

"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.

The families of some of the victims have arrived in France and were taken to the area of the crash site today.

The retrieval of one of two cockpit recordings came as French President Francois Hollande, Germany's Angela Merkel and Spain's Mariano Rajoy travelled to the crash site yesterday to pay tribute to the 150 victims, mostly German and Spanish.

The last words of the pilot to the ground confirmed the next navigational waypoint, ending with a call-sign and "thank you".

The Lufthansa spokesman said that since the 11 September 2001 attacks, cockpit doors cannot be opened from the outside, in line with regulations.

The second data black box has not yet been found.

A German foreign ministry spokesman said 75 Germans were killed in the crash according to latest information - the first major air passenger disaster on French soil since the 2000 Concorde accident just outside Paris.

Spanish officials said 51 Spaniards were among the victims.

As well as Germans and Spaniards, victims included three Americans, a Moroccan and citizens of Britain, Argentina, Australia, Belgium, Colombia, Denmark, Israel, Japan, Mexico, Iran and the Netherlands, officials said.

However DNA checks to identify them could take weeks, the French government said.

Police and forensic teams on foot and in helicopters investigated the crash site about 100km north of Nice.

"When we go to a crash site we expect to find part of the fuselage. But here we see nothing at all," pilot Xavier Roy, coordinating air operations, said of the confetti of debris.

Mr Roy said teams of investigators had been dropped by helicopter onto the site and were working roped together at altitudes of around 2,000 metres.