The personality of the co-pilot suspected of deliberately flying a Germanwings passenger jet into a mountainside in the Alps is a "serious lead" in the inquiry but not the only one, a French investigator says.

"We have a certain number of elements which allow us to make progress on this lead, which is a serious lead but which can't be the only one," police chief Jean-Pierre Michel told AFP in the western German city of Duesseldorf on Saturday.

The investigation so far has not turned up a "particular element" in the life of co-pilot Andreas Lubitz which could explain his alleged action in the ill-fated Airbus plane, he added.

All 150 people on board were killed in Tuesday's crash.

The black box voice recorder indicates that Lubitz, 27, locked his captain out of the cockpit of the Germanwings jet and deliberately flew Flight 4U 9525 into a mountainside, French officials said.

German prosecutors believe Lubitz hid an illness from his airline.

They said on Friday that searches of his homes netted "medical documents that suggest an existing illness and appropriate medical treatment", including "torn-up and current sick leave notes, among them one covering the day of the crash".

They did not specify the illness but media reports say he was severely depressed.

The French investigator is part of a three-strong team which has travelled to Duesseldorf to cooperate with German authorities.

"We're going to try to understand what in his life could have led him to the carrying out of the act," Michel said.

Lubitz's ex-girlfriend, identified only as Maria W, has told German newspaper Bild he once said to her "everyone will know my name and remember".

She said Lubitz worried that health problems would dash his dreams and vowed to one day do something to "change the whole system".

If Lubitz did deliberately crash the plane, it was "because he understood that because of his health problems, his big dream of a job at Lufthansa, of a job as captain and as a long-haul pilot was practically impossible", the 26-year-old told Bild.

She said she split up with him because it became "increasingly clear he had problems".