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.

Drawing from a growing bag of tricks, Islamic State accomplices located in Syria likely used phones and WhatsApp accounts belonging to Abaaoud and other attackers to mask the group’s travel to Europe, said a Western security official: “We relied too much on technology. And we lost track.”

Terror attacks in Europe, which have killed more than 200 people in the past 20 months, reflect new operational discipline and technical savvy by the Islamic State terrorists who carried them out, security officials said.

The extremist group’s communications, once commonly conducted on phones and social media accounts easily tracked by authorities, have evolved into a mix of encrypted chat-app messages over WhatsApp and Telegram, face-to-face meetings, written notes, stretches of silence and misdirection.

These techniques helped protect attackers from Western intelligence agencies by leaving few electronic clues in a sea of intercepted data.

In recent months, Europe has been convulsed by a string of simple yet lethal attacks. Some were committed by people who appear to have received little direct training from Islamic State. The suspects in a failed plot in France last week were “remotely controlled” from Syria by the group, prosecutors said Friday. Officials worry such attacks could be a way to distract intelligence services while militants prepare more complex plots.

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