Previously undisclosed Snowden documents detail how eavesdropping, British intelligence guide U.S. drone strikes

British intelligence documents provided to The New York Times and The Guardian by former National Security Agency contractor Edward Snowden reveal telling details about a 2012 drone air strike in Yemen and offer clues to how the U.S. determines terrorist targets. Additionally, the documents appear to show that the N.S.A. has worked in Pakistan and Yemen with the British Government Communications Headquarters, or G.C.H.Q.

The British agency appears to have supplied intelligence for a U.S. strike in Yemen that killed Khadim Usamah, a doctor linked to al Qaeda. Usamah reportedly "pioneered using surgically planted explosives" and was thought to be working with the al Qaeda explosives expert behind the thwarted attack on a Detroit-bound plane in 2009 by "underwear bomber" Umar Farouk Abdulmutallab. British intelligence believed Usamah was recruited "to experiment with implanting a bomb with no metal parts into the abdomen of a suicide bomber," The New York Times reports.

Additionally, the British documents detail how signal intelligence and eavesdropping play central roles in determining terrorist suspects, as well as where they might be hiding. However, the documents also expose flaws in the program. Smartphones, for example, can be easily tracked by the N.S.A. or the G.C.H.Q., but can also be passed from person to person, leading to mistakes when identifying suspects.

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Agencies also try to determine their suspect's B.D.L., or "bed-down location," when planning a strike, in addition to confirming a target's voice and appearance. Still, there are mistakes: In April, a strike killed two Western aid workers held in Pakistan by al Qaeda; intelligence officials had no idea about their presence.

Press officers for the N.S.A. and the C.I.A. declined to comment to The New York Times.