The co-pilot of a Germanwings jet that crashed in the French Alps increased the airplane's speed before it crashed killing all 150 people on board, according to French investigators.

France's BEA crash investigation agency said on Friday it was still reconstructing the flight, but the chilling new detail from a second 'black box' data recorder appeared to corroborate prosecutors' suggestions that the co-pilot acted deliberately.

"A first reading shows that the pilot in the cockpit used the automatic pilot to put the airplane on a descent toward an altitude of 100 feet," the BEA investigation office said in a statement.

"Then several times the pilot modified the automatic pilot settings to increase the speed of the airplane as it descended," it added.

The data was extracted from a charred and badly dented flight data recorder discovered on the mountainside crash site 6,000 feet above sea level on Thursday.

The box arrived at BEA headquarters outside Paris under police guard on Thursday evening and investigators immediately set to work reconstructing the last moments of Flight 9525.

Sticking to a purely factual reading of initial data, the BEA did not speculate on which pilot is thought to have entered the commands in the autopilot, which is governed by a single set of knobs sitting on a console between the two pilots.

Nor was it immediately clear whether the pilot was deliberately accelerating the plane toward the mountain or changing speed settings according to the altitude, which affects the way speed is measured in the cockpit. – Reuters