Arrested, freed, flagged: How top ISIS operative slipped through the net

(CNN) -- Authorities missed an opportunity a decade ago to stop a key ISIS operative thought to be behind the Paris and Brussels attacks, a former Belgian intelligence officer says.

Instead, Andre Jacob, who at the time was the head of the anti-terrorism unit for Belgian intelligence, says that Oussama Atar, believed by French authorities to be the elusive "Abu Ahmad," was incarcerated in multiple Iraqi detention centers where he became further radicalized.

The Belgian-Moroccan na...