The Department of Homeland Security's mandate to carry out domestic surveillance has been a concern for privacy advocates since the organization was first created in the wake of the September 11 attacks. Now a data leak affecting the DHS's intelligence arm has shed light not just on how the department gathers and stores that sensitive information—including about its surveillance of Americans—but on how it once left that data exposed to thousands of government and private sector workers and even foreign nationals who were never authorized to see it.

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An internal DHS memo obtained by a Freedom o

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