The parents of Andreas Lubitz, the co-pilot of Germanwings flight 4U9525 who is suspected to have caused the plane crash, are being questioned by investigators.

After examining the black box of the plane, authorities now believe that Lubitz, 28, deliberately set the Airbus A320 on its fatal descent.

Lubitz, a German national, was living with his parents in his hometown Montabaur, 100kms from Frankfurt. He also owned another apartment in Düsseldorf, the city where the doomed flight was heading.

Both the residences of Lubitz have been searched by police for clues and have so far ruled out terrorism.

A statement from the prosecutors said the raids were aimed at "finding and securing personal documents to obtain evidence of a possible motive".

The Airbus A320 left the Spanish city of Barcelona carrying 150 people but then went down in the French Alps, leaving no survivors. The homes of all the crew members and passengers are also being searched for DNA verification.

Explaining the final moments during the rapid descent of the plane, prosecutor Brice Robin said Lubitz, who was believed to have been the only pilot inside the cockpit during the crash, remained calm.

"He does not say a single word. Total silence. The co-pilot was alone at the controls," said Robin.

There was no sign of panic and he was said to be breathing normally when the airliner was quickly losing altitude despite the screams from the cabin.

"The victims realised just at the last moment. We can hear them screaming. They were all killed instantly," added the prosecutor.

Friends and employers of the co-pilot have said they had not noticed anything untoward in Lubitz's behaviour.