Prosecutor's office spokesman Agnes Thibault-Lecuivre said the inquiry was launched as an accident investigation, not a terrorism investigation.
She said authorities are "not at all" favoring the theory that the plane was downed deliberately, though the status of the inquiry could eventually change if evidence emerges to that effect.
Investigators decided to start the probe before waiting to analyse the plane's flight data and voice recorders, based on evidence gathered so far, she said, without elaborating.
Search teams have recovered its two flight recorders, but they suffered damage and Egyptian investigators were unable to download information from the so-called black boxes.
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The recorders' memory cards arrived today in Paris, Egyptian investigators said.
Technicians at France's air accident investigation agency, the BEA, will attempt to clean and repair them, and then send them back to Egypt for analysis, the BEA said in a statement Friday.