Weeks before Islamic State militant Abdelhamid Abaaoud led the Nov. 13 terror attacks in Paris, French authorities thought he was holed up in northern Syria. Western Intelligence agencies pursuing Abaaoud had tracked him there using cell-phone location data and other electronic footprints.

The Paris attacks, which killed 130 people, showed how badly they were fooled. Abaaoud had slipped past the dragnet and entered the city unnoticed.

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