A U.S. drone killed an American and an Italian held hostage in a January attack on an al Qaeda compound in Pakistan, sparking new questions about the use of the controversial and still-evolving weapon.

The intelligence that underpinned the drone strike turned out to have been tragically incomplete, U.S. officials and lawmakers said Thursday. As a result, American development expert Warren Weinstein and Italian aid worker Giovanni Lo Porto lost their lives after years as captives of the militants.

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