An inert U.S. Hellfire missile sent to Europe for training purposes was wrongly shipped from there to Cuba in 2014, said people familiar with the matter, a loss of sensitive military technology that ranks among the worst-known incidents of its kind.

The unintended delivery of the missile to Cuba has confounded investigators and experts who work in a regulatory system designed to prevent precisely such equipment from falling into the wrong hands, said those familiar with the matter.

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