Abaaoud would find notoriety -- and death -- in November 2015 as the organizer of the terror attacks in Paris. But long before then, he had outmaneuvered Europe's intelligence services, dispatching perhaps dozens of fighters to the continent. He seemed obsessed with opening a new front for ISIS in Europe.
And long before the Paris attacks, Abaaoud had a network in Belgium. He had served time in jail with Salah Abdeslam, who would join him in attacking Paris.
On January 20, 2014, Abaaoud flew from Cologne, Germany, to Istanbul, accompanied by his 13-year-old brother and another would-be jihadist, Yoni Patrick Mayne, also from Molenbeek.
Days earlier, according to intelligence documents obtained by CNN, Abaaoud had received a call from a Turkish mobile number.
The call was from Mehdi Nemmouche, a French-Algerian who had left to fight jihad in Syria in 2012. Radicalized in jail in France, Nemmouche had become a guard and torturer of foreign hostages in Syria, according to one of the few to survive, French journalist Nicolas Henin.
What was said is unknown, but the call confirmed a link between the two. Weeks later, Nemmouche would head back to Europe. He used an elaborate route to disguise his travel from Syria, via Malaysia, Singapore and Hong Kong, before arriving in Frankfurt on March 18, 2014.
German authorities notified the French but, perhaps already sensing Europe's weak security coordination, Nemmouche went to Belgium. And on May 24, he allegedly staged a gun attack on a Jewish museum in Brussels that killed four people. When he was arrested in the French port of Marseille a week later, Nemmouche was in possession of a Kalashnikov rifle and a flag with ISIS rubric. He is still awaiting trial.
Nemmouche was the first among French and Belgian ISIS fighters -- some encouraged and directed by Abaaoud -- to stage an attack in Europe. At the beginning, it was a trickle but gradually it became a stream.
ISIS began developing encryption techniques and producing high-quality fake documents for its Europe-bound fighters. Flights were eschewed for sea and land routes that would exploit the migrant flows already stretching European frontier agencies in the summer of 2014.
The exact equation between 'freelancers' or 'lone wolves' inspired by ISIS and those trained and directed by the group remains elusive. Unlike the more hierarchical al Qaeda, ISIS gives its operatives much more autonomy.
But by the autumn of 2014, Abaaoud's determination to export terror -- likely endorsed by other French jihadists such as Fabien Clain and Salim Benghalem -- appears to have found an audience among the ISIS leadership. The change in sentiment was probably because of the beginning of coalition airstrikes against ISIS -- first to defend the Yazidi population in Iraq in August and then to support Syrian Kurds fighting ISIS in Kobani the following month.
It was then that ISIS spokesman and leading ideologue Abu Mohammed al-Adnani issued a threat to the 'Crusaders.'
'O Americans, O Europeans, the Islamic State did not initiate a war against you,' Adnani said. 'It is you who started the transgression against us, and thus you deserve blame and you will pay a great price.'
Adnani made this threat to Europeans. 'You will pay the price as you walk on your streets, turning right and left, fearing the Muslims. You will not feel secure even in your bedrooms. We will strike you in your homeland, and you will never be able to harm anyone afterwards.'
By then, according to European counterterrorism officials, some 300 European jihadists were already back from Syria. And security services were struggling to keep up with the inflow.
One official acknowledged to CNN, even before airstrikes began, that 'the threat of attacks has never been greater -- not at the time of 9/11, not after the war in Iraq -- never.'
He and others envisaged a flood of small-scale, solo attacks. But he added: 'The worry is that Europeans in the group may out of their own initiative return home to launch attacks. The question becomes to what extent will the ISIS leadership try to control this?'
The answer would be: to a great extent. As airstrikes intensified, taking out key leaders of ISIS and eroding its control of territory, so did the group's desire for revenge. What was different was the ambition: no longer opportunistic and individual but complex and directed.
Despite an international arrest warrant for terrorism, Abaaoud left Syria at the end of 2014 and sneaked into Greece, moving between apartments in Athens. From there he began to plan attacks in Europe. He later boasted in the ISIS online magazine Dabiq that he had traveled as far as Belgium with two men who were later killed in a shoot-out with Belgian police.
'We spent months trying to find a way into Europe,' he told Dabiq ... 'We succeeded in finally making our way to Belgium. We were then able to obtain weapons and set up a safe house while we planned to carry out operations against the crusaders.'
The Belgian security services had monitored calls from Greece to relatives of men planning the attacks in Belgium, before surrounding their safe-house in the town of Verviers on January 15.
Two days later, as European intelligence agencies scrambled to find Abaaoud, Greek police raided two apartments in central Athens. They found Abaaoud's fingerprints but no trace of the man himself. They did, however, according to sources cited by Le Parisien newspaper, discover a computer which contained bomb-making instructions and attacks plans.
Intelligence sources say Abaaoud slipped back into Syria to step up the training of foreign fighters for attacks in Europe. There was no shortage of French and Belgian fighters in and around Raqqa who had the language, contacts and local knowledge to survive in Europe.
Many ISIS members who had gone to Syria had criminal records; they were street-smart and had long-established underworld contacts in Europe. They knew how to take advantage of borderless Europe even as intelligence services were still in national silos.
There were other factors working in favor of Abaaoud's plans: the Turkish border was relatively open. ISIS had secured a large supply of blank Syrian passports. It also had plenty of cash.
Another Abaaoud associate was a 29-year-old Frenchman, Reda Hame. According to a French investigation document obtained by CNN, Abaaoud provided him with training in Raqqa and pushed him to return to France 'to open fire at a concert or in a crowded place where he could take hostages and then stage a suicide attack when police arrived.'
Hame traveled via Prague in June 2015 but was arrested soon after he reached France and began cooperating. He provided information on the weapons training he'd received and the encryption software he'd been given to communicate.
According to a transcript of his interrogation obtained by Le Monde, Hame said ISIS was now focused on exporting terror to Europe.
'All I can tell you is that this will happen very soon. It was a real factory over there, and they are really trying to hit France and Europe. '
By then, European intelligence services were beginning to fear exactly that. But they had inadequate specific, actionable intelligence. There was no cascade of orders to intercept, no obvious money trail. And despite many promises, improving Europe-wide intelligence coordination was lagging.
While Hame was being questioned in early August, the cell that would attack Paris three months later and its support structure was already being infiltrated into Europe. On August 4, Salah Abdeslam left a Greek port for Italy in the company of Ahmed Dahmani, who would later be detained in Turkey after leaving Amsterdam the day after the Paris attacks.
How many others may have slipped in by land and sea from Syria?
Most of the Paris attackers for sure, and at least one of the Belgian bombers, as well as an associate of Abdeslam arrested with him in Molenbeek two weeks ago.
Another of those travelers may have been the young man who left Cologne with Abaaoud back in January 2014. Last year, ISIS members posted photos showing the apparently lifeless corpse of Yoni Patrick Mayne in Syria.
His mother insists he was killed. The Belgian newspaper Derniere Heure reports that investigators apparently have other pictures of his body than those shared on social media, which were identified by Mayne's family. And yet a European security bulletin issued after the Brussels attacks and obtained by CNN listed Mayne as one of eight men still wanted in connection with the Paris and Brussels attacks. As the saying goes, perhaps he's not dead yet.
The inside story of the Paris and Brussels attacks
And there may be dozens of other ISIS operatives across Europe planning further episodes in the campaign driven by Abaaoud.
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