The death in Paris of the suspected terror attack ringleader - a high-profile jihadi - prompted fears Thursday that a major intelligence breakdown may have allowed him to pursue his murderous plot.
Abdelhamid Abaaoud, 27, was killed during an explosive raid on an apartment in Saint-Denis, according to the Paris prosecutor's office. His body was riddled with bullet wounds and he was identified by his fingerprints, officials said Thursday as they announced his death.
Confirmation that Abaaoud was in Paris will put pressure on European security services. Until last week's massacre, he was widely assumed to be in Syria.
"This is a major failing," Roland Jaquard, at the International Observatory for Terrorism, told Reuters.
Even before last Friday's attacks, Abaaoud had emerged as Belgium's most notorious jihadist, a zealot so devoted to the cause of holy war that he recruited his 13-year-old brother to join him in Syria.
French officials have linked Abaaoud to at least four out of six foiled attacks this year.
It was not immediately clear how Abaaoud was able to travel through Europe despite being known to international intelligence agencies,
"The mind boggles," said counter-terrorism analyst Simon Palombi, a consultant for the International Security Department at British think tank Chatham House. "The only way I can understand it is if an intelligence official had reviewed them and [he] simply was not deemed a risk."
"The fact that [Abaaoud and the other attackers] weren't discovered and weren't disrupted suggests that they have a big issues with their security apparatus," he added.
One challenge faced by security services across Europe is that known terrorist ringleaders - a relatively small group in number - often depend on a wider network of sympathizers or friends who are aren't as easy to track, according to Raffaello Pantucci, director of International Security Services at the Royal United Services Institute in London.
“Clearly there are support networks that people can use to go back and forth, " he said. "If you look at every case that has been disrupted the terrorists clearly have a network of people and places that they can go to."
Belgian authorities also suspect Abaaoud, who grew up in the Brussels neighborhood of Molenbeek in a Moroccan immigrant family, of helping organize and finance a terror cell in the city of Verviers that was broken up in an armed police raid on Jan. 15.
The jihadi, who boasted about evading Western intelligence, is also tied to the thwarted attacks on a Paris-bound high-speed train and a church near the French capital earlier this year. He was even the subject of European and Belgian arrest warrants, according to French Interior Minister Bernard Cazeneuve.
A big problem facing European nations scrambling to screen out extremists is that many of the continent's frontier borders are porous and many others do not carry out passport checks under the terms of the Schengen treaty allowing freedom of movement between 26 European Union member states.
It prevents the kind of intensive security checks administered by the U.S. Department of Homeland Security.
Greece has hundreds of islands off its coast, providing a multitude of options for those attempting to reach the West by sea. Almost 750,000 migrants and refugees have made it to Europe this year — including nearly 220,000 people who arrived by sea in October, according to the United Nations refugee agency (UNHCR).
"We are fundamentally different [to the U.S.] in that regard," said Nick Whitney, the co-director of the European power program at the European Council on Foreign Relations. "Even if we wanted to go down that intensive Homeland Security route it simply isn't possible because of geography."
France's Interior Minister Bernard Cazaneuve hinted at these challenges when he said it was "urgent for Europe to come together to defend itself against terrorism."
He added: "The [European Union] needs a plan and needs its borders reinforced."