PARIS — The prime contractor of the European Vega rocket that failed in July, resulting in the loss of a United Arab Emirates reconnaissance satellite insured for $413 million, said a manufacturing defect in the rocket’s second-stage motor might be the root cause of the failure.

A European government board of inquiry concluded that a “thermal/structural” failure of the rocket’s Z23 second stage motor’s forward dome is the most likely cause. But the board’s Sept. 5 statement on the preliminary results of its inquiry was too vague to . . .

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