French investigators believe Abdelhamid Abaaoud, the jihadist killed in a police raid on Wednesday, organised last week's Paris attacks which killed at least 129 people.
Abdelhamid Abaaoud, a Belgian national, grew up in Molenbeek, a district of Brussels known for its many Arab immigrants and blighted by high unemployment.
The 28-year-old was an associate of attacks suspect Salah Abdeslam, who is on the run and whose brother Brahim blew himself up in Paris on 13 November.
Brahim died in the Comptoir Voltaire bar, without killing others, while at least six other jihadists carried out deadly suicide attacks elsewhere. They are all believed to have been recruited by the Islamic State militant group (IS).
Abaaoud had been implicated in four out of six foiled attacks since this spring in France and sentenced to 20 years in prison in absentia, French Interior Minister Bernard Cazeneuve said.
He added that all the thwarted plots had the same modus operandi, saying the "planning from abroad of a violent act by jihadists (was) from European countries, trained in the use of weapons and then sent to our territory to carry out the attacks".
An international arrest warrant was also issued for Abaaoud, Mr Cazeneuve said.
Abaaoud - alias Abu Umar al-Baljiki - is believed to have joined IS in early 2013.
There was confusion about his whereabouts prior to this death, with IS claiming he was in Syria.
The group may have lied about his location to divert attention from him following the Charlie Hebdo attacks and the Verviers raid.
Jihadists often mislead intelligence agencies about the whereabouts of their militants, invoking the concept that "war is deception".
But his presence in France raises big questions about security failings in France the the 28-member European Union.
"No information from European countries he could have passed through before arriving in France was communicated to us," Mr Cazeneuve said.
He added that it was only on 16 November - three days after the attacks - that "intelligence services of a country outside Europe indicated they had knowledge
of his presence in Greece," he said, without saying which country.
It is not clear when Abaaoud became radicalised. The Associated Press reports that he attended one of Belgium's top secondary schools - Saint-Pierre d'Uccle.
He was in contact with Mehdi Nemmouche, a jihadist of Franco-Algerian origin, who shot and killed four people at the Jewish Museum in Brussels in May 2014.
Nemmouche had also spent time in Molenbeek - an area where, Belgian officials admit, the radical Salafist ideology has flourished among some young Muslims.
In recent years, Molenbeek has had "the highest concentration of foreign terrorist fighters in Europe", said Liesbeth van der Heide, at Leiden University's Centre for Terrorism and Counterterrorism in the Netherlands.