The chilling new detail from the BEA agency is based on an initial reading of the plane's "black box" data recorder, found blackened and buried at the crash site yesterday.
It strengthens investigators' initial suspicions that co-pilot Andreas Lubitz intentionally destroyed the plane -- though prosecutors are still trying to figure out why.
All 150 people aboard Flight 9525 from Barcelona to Duesseldorf were killed in the March 24 crash.
The agency says it will continue studying the black box for more complete details of what happened. The Flight Data Recorder records aircraft parameters such as the speed, altitude, and actions of the pilot on the commands.
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German prosecutors have said Lubitz's medical records from before he received his pilot's license referred to "suicidal tendencies," and Lufthansa, Germanwings' parent company, said it knew six years ago that Lubitz had had an episode of "severe depression" before he finished his flight training.
In Marseille, prosecutor Brice Robin said that his investigation focuses on France for now, but he has filed a formal request for judicial cooperation from Germany that could expand the scope of his probe.
"It's a voluntary action that guided this plane toward the mountain, not only losing altitude but correcting the aircraft's speed," he said yesterday.