The pilot at the controls of a Germanwings jet that crashed in the French Alps accelerated the plane into the mountainside, killing all 150 people on board, according to French investigators.

France's BEA crash investigation agency declined to confirm growing evidence against the co-pilot before completing its own analysis, but the chilling new detail from a newly recovered second 'black box' seemed to corroborate prosecutors' claims that he killed himself and everyone else deliberately.

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

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

Based on cockpit audio recordings from the first black box, recovered hours after the March 24 crash, prosecutors believe 27-year-old German co-pilot Andreas Lubitz locked the captain out of the cockpit and veered the plane into a descent.

As he barricaded himself at the Airbus A320's controls, he appears to have ignored efforts by his senior colleague to bash his way through the cockpit door of the 24-year-old jet, which had been reinforced under rules to protect pilots after the Sept. 11, 2001 attacks in the United States.

The latest evidence on the plane's speed was extracted from a charred and badly dented flight data recorder discovered among wreckage on Thursday, nine days after the worst aviation disaster in France since the 2000 Concorde crash.

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 to Duesseldorf.

The BEA's purely factual description of the initial data focused on the autopilot, which is governed by a set of knobs sitting on a console between the two pilots.

But the brief actions described by the BEA lent technical weight to a picture painted by prosecutors of methodical and cool-headed actions as the aircraft hurtled for eight minutes towards the ground under the control of a trained pilot.

"The autopilot was reprogrammed several times to increase the speed during the descent," a BEA spokeswoman said.

Analyzing data

The flight data recorder monitors hundreds of parameters, including any commands made directly from the co-pilot's seat.

As well as inputs to shared controls such as the autopilot, it may record seat movements that might prove which pilot remained at the controls while the other left the cockpit.

The BEA now faces the laborious task of synchronising the flight data extracted from thermally protected memory strips with the earlier cockpit audio recordings and radar tracks.

The agency's spokeswoman said it would "days or even weeks" to finish analysing the data.

The BEA, whose mandate is to prevent future accidents rather than attribute blame, signalled its intention earlier this week to complete its own safety investigation and deliver a final report, whatever the outcome of separate judicial probes.

It said it would focus on cockpit door mechanisms and pilot screening, in a sign that it could issue recommendations that some experts say may prompt a rethink on regulation worldwide.

Lubitz's motives remain a mystery but investigators have uncovered evidence that he had made suicide preparations.

German prosecutors said on Thursday that Lubitz had made Internet searches on ways to commit suicide in the days ahead of the crash as well as searches about cockpit doors and safety precautions.

Germanwings parent Lufthansa has said Lubitz told its flight school in 2009 he had gone through a period of severe depression. As part of their crash probe, German prosecutors searched the offices of five doctors whose help Lubitz had sought, Der Spiegel magazine reported on Friday.