Search efforts to try to locate the second black box from the downed Germanwings plane continued today as helicopters flew to and from the crash site in the French Alps.

So far investigators have only retrieved the cockpit voice recordings from one of the plane's black boxes, which they say show that the co-pilot, Andreas Lubitz, locked himself alone in the cockpit, as his fellow pilot desperately tried to break in, and caused the airliner to crash.

A chief French investigator said yesterday it was too early to rule out other explanations for the crash.

The second black box contains flight data, which is needed to rule out any mechanical problems with the jet.

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