EXPLAINER: A look at the missile that killed al-Qaida leader

For a year, U.S. officials have been saying that taking out a terrorist threat in Afghanistan with no American troops on the ground would be difficult but not impossible. Last weekend, the U.S. did just that -- killing al-Qaida leader Ayman al-Zawahri with a CIA drone strike. Other high-profile airstrikes in the past had inadvertently killed innocent civilians. In this case, the U.S. carefully chose to use a type of Hellfire missile that greatly minimized the chance of other casualties. Although U.S. officials have not publicly confirmed which variant of the Hellfire was used, experts and others familiar with counterterrorism operations said a likely option was the highly secretive Hellfire R9X -- know by various nicknames, including the "knife bomb" or the "flying Ginsu."