It was an innocuous e-mail, one of millions sent every day by spouses with updates on the situation at home. But this one was of particular interest to the National Security Agency and contained clues that put the sender's husband in the crosshairs of a CIA drone. Days later, Hassan Ghul ... was killed by a drone strike in Pakistan's tribal belt. Documents provided ... by former NSA contractor Edward Snowden confirm his demise in October 2012 and reveal the agency's extensive involvement in the targeted killing program that has served as a centerpiece of President Obama's counterterrorism strategy. The documents provide the most detailed account of the intricate collaboration between the CIA and the NSA in the drone campaign. [The] collection of records in the Snowden trove [make] clear that the drone campaign - often depicted as the CIA's exclusive domain - relies heavily on the NSA's ability to vacuum up enormous quantities of e-mail, phone calls and other fragments of signals intelligence, or SIGINT. To handle the expanding workload, the NSA created a secret unit known as the Counter-Terrorism Mission Aligned Cell, or CT MAC, to concentrate the agency's vast resources on hard-to-find [targets]. Former CIA officials said the files are an accurate reflection of the NSA's contribution to finding targets in a campaign that has killed more than 3,000 people [in] Pakistan.