The co-pilot of doomed Germanwings Flight 9525 repeatedly accelerated the plane as he used its autopilot to crash the A320 in the French Alps, killing himself and 149 others on board, the French air accident investigation agency said Friday.

The BEA said in a statement that an initial reading of the flight data recorder shows that Andreas Lubitz used the automatic pilot mechanism to put the plane into a descent and then repeatedly adjusted it to increase the jet's speed as it crashed into the mountains March 24.

The data recorder was found blackened and buried at the crash site Thursday.

The information strengthens investigators’ belief that Lubitz intentionally destroyed the plane, though prosecutors are still trying to figure out why. The agency says it will continue studying the black box for more complete details of what happened.

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 had spent time researching suicide methods and cockpit door security in the week prior to the plane crash, prosecutors said Thursday. It was the first evidence that the fatal descent may have been a premeditated act.

German prosecutors have said Lubitz’s medical records from before he obtained his pilot’s license referred to “suicidal tendencies.” Lufthansa, Germanwings’ parent company, said it knew six years ago that Lubitz had an episode of “severe depression” before he finished his training.

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

The Associated Press contributed to this report.