The would-be suicide bomber dispatched by the Yemen branch of Al Qaeda last month to blow up a United States-bound airliner was actually a double agent who infiltrated the terrorist group and volunteered for the suicide mission, American and foreign officials said Tuesday. In an extraordinary intelligence coup, the agent left Yemen, traveling by way of the United Arab Emirates, and delivered both the innovative bomb designed for his air attack and critical information on the group’s leaders to the C.I.A., Saudi and other foreign intelligence agencies. After spending weeks at the center of the terrorist network’s most dangerous affiliate,...