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By Richard Engel and Robert Windrem
NBC News

The CIA did not always know who it was targeting and killing in drone strikes in Pakistan over a 14-month period, an NBC News review of classified intelligence reports shows.

About one of every four of those killed by drones in Pakistan between Sept. 3, 2010, and Oct. 30, 2011, were classified as "other militants,” the documents detail. The “other militants” label was used when the CIA could not determine the affiliation of those killed, prompting questions about how the agency could conclude they were a threat to U.S. national security.

The uncertainty appears to arise from the use of so-called “signature” strikes to eliminate suspected terrorists -- picking targets based in part on their behavior and associates. A former White House official said the U.S. sometimes executes people based on “circumstantial evidence.”

Three former senior Obama administration officials also told NBC News that some White House officials were worried that the CIA had painted too rosy a picture of its success and likely ignored or missed mistakes when tallying death totals.

NBC News has reviewed two sets of classified documents that describe 114 drone strikes over 14 months in Pakistan and Afghanistan, starting in September 2010. The documents list locations, death and injury tolls, alleged terrorist affiliations, and whether the killed and injured were deemed combatants or non-combatants.