When the family of Abdelhamid Abaaoud received word from Syria last fall that he had been killed fighting for the Islamic State, it rejoiced at what it took to be excellent news about a wayward son it had come to despise.
Their prayers and the hopes of Western security officials were not answered. Mr. Abaaoud, then 26, was in reality on his way back to Europe to meet secretly with Islamic extremists who shared his determination to spread mayhem. He has since been linked to a string of terrorist operations that culminated with Friday’s attacks in Paris.
“Of course, it is not joyous to make blood flow. But, from time to time, it is pleasant to see the blood of disbelievers,” Mr. Abaaoud declared in a French-language recruiting video for the Islamic State released shortly before his supposed death.
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During his travels back to Europe at the end of last year, European security services picked up his trail and tracked his cellphone to Athens, according to a retired European military official. But they lost him, and soon after that he appeared to have made it back to Belgium, where he had grown up in a moderately successful family from Morocco.
At about the time Mr. Abaaoud began his return journey to Europe, Abu Muhammad al-Adnani, a leader with the Islamic State — he now has a $5 million bounty on his head, offered by the United States — made an impassioned plea for the killings of disbelievers. “If you can kill a disbelieving American or European, especially the spiteful and filthy French, then rely on Allah and kill him in any manner or way however it may be,” Mr. Adnani said in a recorded message.
Western intelligence agencies, worried that the Islamic State was planning to widen its carnage from the Middle East to Europe, tried to track a slow but steady trickle of fighters in Syria as they headed home to the Continent.
A Belgian television station reported Monday that security services had been alerted to Mr. Abaaoud’s return to Europe by a telephone call he made from Greece to an inmate, the brother of a known jihadist, in a Belgian prison.
The realization among security officials that Mr. Abaaoud was back in Europe led to a major operation to intercept him. A safe house for militants he had helped set up in eastern Belgium was raided in January.
There, two of his comrades, including the brother of the inmate he had called, were killed. Belgian authorities trumpeted the raid as having thwarted “a major terrorist operation.”
But it missed its principal target, Mr. Abaaoud, who then somehow made his way back to Syria, which the Islamic State calls “Sham.”
“Allah blinded their vision and I was able to leave and come to Sham despite being chased by so many intelligence agencies,” he later told Dabiq, a slickly produced magazine published by the Islamic State.
It is not known whether Mr. Abaaoud had any direct contact in Syria with Mr. Adnani, the architect of what the C.I.A. director, John O. Brennan, on Monday called the Islamic State’s “external operations agenda.” As a low-level fighter, Mr. Abaaoud was unlikely to have mixed with senior figures in the militant group’s hierarchy, experts in Belgium said.
But the two men shared a passion for propaganda, with Mr. Adnani serving as the Islamic State’s official spokesman and Mr. Abaaoud featured in various recruitment campaigns.
Mr. Abaaoud also had an invaluable asset for Islamic State leaders eager to take their battle to Europe — a pool of friends and contacts back home willing to carry out attacks.
As a young man, like many of the jihadists who have carried out attacks in Europe, including the brothers who attacked the French satirical magazine, Charlie Hebdo, in January, Mr. Abaaoud showed far more interest in thievery and drugs than Islam, particularly the highly disciplined, self-sacrificing Salafi strain favored by many militants.
Nor was his family impoverished. His father, Omar, owned a clothing store off the market square in Molenbeek, a borough of Brussels, and the family lived nearby in a spacious if shabby corner home on Rue de l’Avenir — Future Street — near the local police station.
Despite his subsequent denunciations of the mistreatment suffered by Muslims in Europe, he enjoyed privileges available to few immigrants, including admission to an exclusive Catholic school, Collège Saint-Pierre d’Uccle, in an upscale residential district of Brussels.
He was given a place as a first-year student in the secondary school but stayed only one year. An assistant to Saint-Pierre’s director, who declined to give her name, said he had apparently flunked out. Others say he was dismissed for poor behavior.
He then drifted into a group of friends in Molenbeek that engaged in various petty crimes. Among his friends were Ibrahim and Salah Abdeslam, two brothers who, like Mr. Abaaoud, lived just a few blocks away and are now at the center of the investigation into the Paris attacks.
Ibrahim Abdeslam was one of the suicide bombers on Friday and Salah Abdeslam, who rented a car in Brussels that was used to transport some of the gunmen in the attacks on Paris, is the target of an extensive manhunt.
Mr. Abaaoud was arrested for petty crime in 2010 and spent time in the same prison in Brussels where Ibrahim Abdeslam was being held, according to the spokesman for Belgium’s federal prosecutor and Ibrahim’s former lawyer. It is not known if they were in touch while in the prison, but they did not stay long. After their release, they returned to Molenbeek, often hanging out at a dingy bar known as a hang-out for drug dealers.
To the dismay of his family, which had not seen him show any religious zeal, Mr. Abaaoud suddenly moved to Syria in the beginning of 2014, according to jihadi experts tracking Belgian militants.
Soon after his arrival in Syria, where he stayed for a time in a grand villa in Aleppo used to house French-speaking jihadists, he explained his choice in a video: “All my life I have seen the blood of Muslims flow. I pray that God breaks the backs of those who oppose him” and “that he exterminates them,” he said.
Early this year, the French magazine Paris Match found a film that showed Mr. Abaaoud grinning and making jokes as he dragged corpses with a pickup truck.
“I suddenly saw my picture all over the media,” he told Dabiq, adding that “thanks to Allah, the infidels were blinded by Allah” and did not spot him when he returned to Europe at the end of last year.
He also somehow persuaded his younger brother, Younes, who was still in Molenbeek and only 13 to join him in Syria. The boy left Belgium for Syria on his own without raising any suspicion from the authorities.
Mr. Abaaoud’s father, Omar Abaaoud, joined a state prosecutor’s case against his son in May for having recruited Younes.
“I can’t take it anymore,” Mr. Abaaoud told local reporters at the time. “I am on medication,” he said, adding that his son “dishonoured our family. He destroyed our families. I don’t ever want to see him again.”
His father is now living in Morocco and wants to put the property on Future Street up for sale, a family friend said.
Now, Mr. Abaaoud is suspected of being a leader of a branch of the Islamic State in Syria called Katibat al-Battar al Libi, which has its origins in Libya. This particular branch has attracted many Belgian fighters because of language and cultural ties, said Pieter van Ostaeyen, who tracks Belgian militants.
Many Belgian Muslims are of Moroccan origin, he said, and speak a dialect found in eastern Morocco that is similar to a Libyan dialect. Aymenn Jawad al-Tamimi, who studies jihadi groups at the Middle East Forum, a research center in Washington, said there was no evidence yet that the Paris attacks had been ordered by Mr. Adnani or the Islamic State’s overall leader, Abu Bakr al-Baghdadi.
But he added that the soldiers at Libyan branch that includes Mr. Abaaoud has played a prominent role in exporting violence. One of their tasks he said, has been to organize plots that “involved foreign fighters, sleeper cells in Europe that were connected with an operative inside of Syria and Iraq, usually in a lower to midlevel position.”
About 520 Belgian fighters have gone to Syria or Iraq to fight, making Belgium the biggest suspected source on a per capita basis of foreign fighters for ISIS. According to posts on Twitter and other social media accounts, the two men who were killed during the raid in Verviers, Belgium, in January, were members of Katibat al-Battar.
Bilal Hadfi, one of the Paris attackers, was in contact through Facebook with members of Katibat al-Battar, which means “Sword of the Prophets,” and particularly one fighter, who went by the nickname Abu Islaeyem.
Mr. Hadfi, who until spring 2014 made Facebook posts about soccer, became an eager militant in a matter of months. He left for Syria early this year and in June posted a call for an attack on the West on Facebook.
“Those dogs are attacking our civilians everywhere,” he wrote. “Strike them in their community of pigs so they can’t feel safe again in their own dreams.”