PARIS --- The co-pilot of the doomed Germanwings flight repeatedly sped up the plane as he used the automatic pilot to descend the A320 into the Alps, the French air accident investigation agency said Friday.

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 Thursday.

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 BEA said the preliminary reading of the data recorder shows that the pilot used the automatic pilot to put the plane into a descent and then repeatedly during the descent adjusted the automatic pilot to speed up the plane.

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.

Based on recordings from the plane's other black box, the cockpit voice recorder, investigators say Lubitz locked the pilot out of the cockpit and deliberately crashed.

Lubitz spent time online researching suicide methods and cockpit door security in the week before crashing Flight 9525, prosecutors said Thursday _ the first evidence that the fatal descent may have been a premeditated act.

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.

Robin underlined French investigators' conviction that he was conscious until the moment of impact, and appears to have acted repeatedly to stop an excessive speed alarm from sounding.

"It's a voluntary action that guided this plane toward the mountain, not only losing altitude but correcting the aircraft's speed," he said Thursday.

The mountain rescue officer who found the data recorder, Alice Coldefy, described Friday the unexpected discovery in a spot that had already been repeatedly searched.

"I found a pile of clothes, we were searching it, we were moving them downhill and while doing this I discovered a box. The color of the box was the same as the gravel, of the black gravel, that is everywhere at the crash site," she told reporters in Seyne-les-Alpes.

So-called black boxes are actually orange, but this one had burned up in the crash and blended with the dark earth covering the area, known to local guides as "the black lands."

"I didn't realize I had found it and I wasn't thinking it was possible to find it among all this debris," she said.

Mountain officers and trained dogs are continuing to search the site. When the terrain is fully cleared of body parts and belongings, a private company will take out the large airplane debris.