In five attacks from 2012 to 2015, alleged assailants were born in Europe and ideologically​ motivated. (The Washington Post)

In the aftermath of the fiery siege inside a third-floor apartment in the historic Paris suburb of Saint-Denis, French police found an “impact-riddled” body in the rubble. On Thursday — one day after that pre-dawn raid — French officials confirmed that Abdelhamid Abaaoud was dead.

European intelligence officials believe the Islamic State operative and Belgian national in his late 20s was the ringleader behind the devastating attacks in Paris last week — the worst on French soil since World War II. His terrorist cell, according to a senior French police source familiar with the case, had planned an attack Wednesday among the steel and glass towers of Paris’s financial district. That plot took shape, the source said, after Abaaoud had apparently failed to stage an attack last Friday in the densely populated neighborhood of Montmartre.

His death marked a key victory in the massive counterterrorism operations launched after last Friday’s massacre in Paris. “These people cannot cause any more harm,” French Interior Minister Bernard Cazeneuve said Thursday. “The Europe we love, the Europe we have developed and built, must do everything it can to combat terrorism.”

But it also raised serious questions about the ease with which homegrown Islamist extremists seeking to stage terrorist strikes in Europe are slipping between the battlefields of the Middle East and the cosmopolitan streets of the continent.

What we know so far about who carried out the Paris attacks

The fact that Abaaoud was found in a Paris apartment highlights “a major collective failure” of the European Union, said Jean-Charles Brisard, president of the French Center for Analysis of Terrorism.

Europe’s open-border agreement, allowing freedom of movement among 26 nations, “was conceived against external threats, but today we face internal threats from citizens traveling to Syria and Iraq who come back and pose a threat.” He added, “We are completely blind.”

The Washington Post on Wednesday reported that Abaaoud had died in the raid, citing two senior European officials from different countries who were informed by French authorities about Abaaoud’s death. On Thursday, Agnés ­Thibault-Lecuivre, spokeswoman for the French prosecutor’s office, said that based on gathered intelligence, officials “thought” Wednesday that the dead body could be Abaaoud. An announcement on Thursday, she said, came after conclusive forensic evidence from a fingerprint analysis.

A series of tips and lucky breaks led to Abaaoud’s door Wednesday. One of the most important was a diamond in the trash — a discarded cellphone found in a garbage can near the Bataclan concert hall that had been stormed by three terrorists last Friday. It belonged to one of the attackers and contained a message that police now believe was sent to Abaaoud shortly before the operation started: “It’s on. We’re starting.”

But French authorities said a surprising tip they received several days after the attacks also aided them. The tipster said that Abaaoud was not in the deserts of the Middle East but in the suburb of Saint-Denis. Another clue came from their phone surveillance of Hasna Aitboulahcen, a 26-year-old woman linked to Abaaoud who would eventually die during the raid Wednesday.

She did not talk to Abaaoud over the phone, but she did receive a call from someone who said her “cousin” was coming. Police do not believe that Abaaoud was in fact related to Aitboulahcen, but the police source said that the caller referred to him that way. Two senior European officials also said Moroccan authorities played a key role in providing information that led to the apartment.

When police arrived at the Rue du Corbillon apartment about 4 a.m. Wednesday, they detonated an explosive designed to blow open the door. But the door, heavily reinforced in a way that suggested those inside were braced for an attack, was only partially destroyed in the blast.

Police set off a second explosive, but the delay gave the militants time to arm themselves. So when police stormed in, those inside were ready.

A Malinois police dog went in first as a scout, only to be shot by powerful hunting munition.

Police believe a police sniper who was positioned on the roof of a nearby building may have killed Abaaoud. The sniper fired at Abaaoud sometime near the beginning of the raid, the police source said.

But a lengthy firefight and a series of explosions that took place inside the small, confined space left Abaaoud’s corpse in a condition that has made it difficult for police to definitively establish how he died. On Thursday, the police source said, Belgium provided Abaaoud’s DNA and fingerprint information for a final analysis.

Police believe the cell tracked to Saint-Denis, also headed by Abaaoud, had been planning an attack on Montmartre, a neighborhood popular with tourists, on Nov. 13, the day of the other attacks. It’s not known why the cell did not carry out the assault.

Yet the big question remained: How did a man believed to be involved in four of six foiled terrorist plots in Europe come to hole up in a Paris apartment?

Even before a European warrant was issued in 2014 for his arrest, officials now believe, Abaaoud had become an architect of the Islamic State’s plans to strike at French-speaking targets in Europe. He has been linked, for instance, to Mehdi Nemmouche, the French national who killed four in a shooting at a Jewish museum in Brussels in May 2014.

Cazeneuve said Thursday that, among the six attacks foiled by French intelligence services since the spring of this year, Abaaoud appears to have been involved in four of them. They include a planned attack on a church in Villejuif just south of Paris in April. He was also tied to Reda Hame, a French foreign fighter arrested in August who told authorities that Abaaoud had asked him to strike targets in France, including “a concert hall.”

On Thursday, Cazeneuve said that a “non-European country” had provided information Monday that Abaaoud had passed through Greece. The police source said that at least one of the three suspects captured alive Wednesday had also been in Greece, again raising the prospect that militants might be taking advantage of the routes into Western Europe being used by hundreds of thousands of asylum seekers fleeing the war-torn Middle East.

Ivo Priebe, spokesman of the German Federal Police, confirmed that Abaaoud was checked by police at Cologne Bonn Airport on Jan. 20, 2014. Priebe would not say where Abaaoud was going. But two security officials familiar with the case said he was traveling to Syria. German authorities took note of his presence and alerted the Belgians. But Priebe said that at that time there were no instructions to arrest him.

“As a Belgian citizen, he enjoyed freedom of movement,” Priebe said.

Karla Adam, Emily Badger, Cléophée Demoustier and Virgile Demoustier in Paris and Brian Murphy in Washington contributed to this report.

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