French investigators said Wednesday they had succeeded in extracting "usable data" from the first black box recovered from the Germanwings Airbus A320 that crashed in the French Alps.

Remi Jouty, head of French air crash investigation agency BEA, said there was still not "the slightest explanation" at this stage on the reasons for the crash.

"We have just succeeded in extracting usable data from the cockpit voice recorder," he said, referring to the black box that records sounds and conversations from the cockpit.

But he said the data had only been retrieved in the last few minutes before his press conference, and investigators had not yet analysed the recordings.

He ruled out the possibility of an explosion, saying "the plane was flying right to the end," but could not put forward any other theories.

"At this stage, clearly, we are not in a position to have the slightest explanation or interpretation on the reasons that could have led this plane to descend... or the reasons why it did not respond to attempts to contact it by air traffic controllers," said Jouty.

He said he was optimistic that the second black box, which records technical flight data, will soon be found given that the debris is spread over a relatively limited area.

French President Francois Hollande said earlier that the casing of the second black box had been found, but not the box itself.