Marseille prosecutor Brice Robin, citing information from the "black box" voice recorder, said the co-pilot was alone in the cockpit.

He intentionally started a descent while the pilot was locked out.

Mr Robin said there was "absolute silence in the cockpit" as the pilot fought to re-enter it.

He said air traffic controllers made repeated attempts to contact the aircraft, but to no avail. Passengers could be heard screaming just before the crash, he added.

Details are emerging of the German co-pilot's past - although his apparent motives for causing the crash remain a mystery.

Mr Lubitz, 27, had undergone intensive training and "was 100% fit to fly without any caveats", according to Carsten Spohr, the head of Lufthansa, the German carrier that owns Germanwings.

Mr Spohr said Mr Lubitz's training had been interrupted for several months six years ago, but did not say why.

The training was resumed after "the suitability of the candidate was re-established", he said.

German Chancellor Angela Merkel told reporters that the co-pilot's apparent actions had given the tragedy a "new, simply incomprehensible dimension".

Police have been searching the co-pilot's home in Montabaur, near Frankfurt, as well as a flat he kept in Duesseldorf.

The Airbus 320 from Barcelona to Duesseldorf hit a mountain, killing all 144 passengers and six crew, after an eight-minute descent.