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This editorial for the Surveillance and Security Intelligence After Snowden issue provides a very brief history of National Security Agency whistleblowers and investigations before Edward Snowden, and sets the current wave of NSA whistleblowing in the context of a growing demand for openness, transparency and accountability opposing the renewed closures and secrecy of the War on Terror. It also provides some links for further research and reading.
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Wood et al. (2015) studied this question.
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