LE BOURGET, France (AP) -- French investigators report they're still "very far" from knowing exactly why an Air France passenger jet plunged into the Atlantic Ocean last month.

After a month-long probe, investigators say they now know the plane was intact when it made a high-speed, vertical dive into the sea. A lead investigator says no traces of fire or explosives have been found.

The first preliminary findings also show that problems with the jet's speed sensors were a factor in the June 1 crash, but not its direct cause.

An investigator says no mayday calls were issued. Life vests found in the wreckage had not been inflated.

The Airbus was en route from Rio de Janeiro to Paris when it went down with 228 people aboard. The plane's black boxes have not been found in the ocean depths, though a search continues until next week.

France's accident investigation agency is calling this one of history's most challenging plane crash investigations.

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