PARIS—The suspected architect of the Paris attacks who was killed in a hail of bullets here Wednesday had traveled through Greece recently, authorities said, raising fresh concerns about Europe’s ability to protect its most vulnerable borders from known threats.
The death of Abdelhamid Abaaoud is a coup for French authorities, who worked from phone records and witness tips to track him to a small apartment building in a Parisian suburb before launching a dramatic raid early Wednesday.
But the ability of the Islamic State operative—who French officials previously believed was in Syria—to slip through borders unchecked could exacerbate an already heated debate among Europeans over how to tighten controls as thousands of migrants enter Europe each week.
“If Europe doesn’t take its responsibilities, then it’s the entire Schengen system that will be in jeopardy,” French Prime MinisterManuel Valls said Thursday, referring to the accord that allows free travel among 26 countries across the continent.
The tide of migrants streaming into Europe has prompted several countries to introduce border checks or build razor-wire fences. Now, in the wake of the Paris attacks, the European Union is considering fast-tracking a raft of new measures, notably to make it mandatory for member countries to share the names of people denied entry and wanted felons.
Mr. Abaaoud’s own crisscrossing through Europe in recent months illustrates the problem.
On Thursday, German officials said that he had flown out of the Cologne-Bonn airport in western Germany in January 2014. Though he was on an EU watch list at the time, the entry didn’t direct authorities to detain him or prevent his exit. Nor was Germany’s domestic intelligence agency, which is responsible for tracking suspected extremists on German soil, notified that he had passed through the airport, the officials said.
Only on Monday—three days after the Paris attacks—did French authorities realize that Mr. Abaaoud was no longer in Syria when an intelligence service outside Europe informed them that they had tracked Mr. Abaaoud’s recent movements in Greece, said French Interior Minister Bernard Cazeneuve without elaborating further.
The Belgian-born terrorist is suspected of having planned and coordinated three teams of attackers who carried out the brazen assaults on the French capital on Friday, using explosives and automatic rifles to kill 129 people. French SWAT teams battled with Mr. Abaaoud and others with suspected terrorist links for hours on Wednesday, firing 5,000 rounds to subdue the group holed up in an apartment.
“The brain or one of the brains—we have to be extremely careful—was found among the dead,” Mr. Valls told parliament on Thursday.
After receiving the intelligence tip that Mr. Abaaoud had been recently spotted in Greece, French police shifted gears, poring over communications data for signs of whether Mr. Abaaoud was in the Paris region. Investigators also began tracking his cousin, Hasna Ait Boulahcen, who they suspected might have been in contact with Mr. Abaaoud.
In the predawn hours of Wednesday, heavily armed police quietly sealed off Mr. Abaaoud’s hideout: an apartment building located on a crime-ridden street of the Saint-Denis suburb north of Paris.
The operative died in a barrage of gunfire and grenades as police fought their way into the apartment and one militant detonated a suicide vest. Investigators are now probing human remains from the scene to determine whether it was Ms. Ait Boulahcen who blew herself up.
Prosecutors said Mr. Abaaoud’s body was so badly damaged in the raid that they couldn’t tell if he, too, had blown himself up or not. Forensic teams worked through the night to identify Mr. Abaaoud’s remains, using his fingerprints.
In killing Mr. Abaaoud, France has deprived Islamic State of a key operative. Mr. Abaaoud combined battlefield experience in Syria with a broad network of associates in Europe that allowed him to mount one of the deadliest terror attacks on European soil in years.
Investigators are probing whether militants linked to Mr. Abaaoud had been planning an additional attack on the Paris neighborhood of Montmartre, police said Thursday. A Renault Clio rented by Salah Abdeslam, a suspect in the attacks who remains at large, was found in the area Friday, Paris prosecutors said.
On Thursday, Mr. Cazeneuve said authorities were investigating whether Mr. Abaaoud was involved in a host of other attacks and plots against Europe. Mr. Cazeneuve said the terrorist was linked to an attack on churches in Villejuif, a Paris suburb, in April that led to the death of a woman.
Three other radicals with suspected links to Mr. Abaaoud have been detained in recent months while traveling to Europe, Mr. Cazeneuve said, including one militant apprehended in August who said he had been trained and assigned by Mr. Abaaoud to carry out an attack in Europe.
French investigators are also looking into whether Mr. Abaaoud was involved in an August attack on a high-speed train traveling between Amsterdam and Paris that was foiled by a trio of American passengers and a Briton.
The Belgian had attained an unusually high rank in Islamic State’s hierarchy for a European. He was military commander, or “emir of war,” in Syria’s eastern Deir Ezzour province, according to local activists.
One of his younger brothers, Yassine Abaaoud, was recently arrested in Morocco on suspicion of terrorist activity and remains in custody, according to a Belgian law-enforcement official. Alexandre Chateau, a lawyer who has represented both brothers, confirmed that Yassine Abaaoud had been arrested in Morocco.
Mr. Chateau declined to comment on the arrest of Yassine Abaaoud, saying he had “suffered” as a result of his older brother’s reputation.
Mr. Chateau also shed light on Abdelhamid Abaaoud’s criminal history, saying he had been repeatedly arrested for crimes, including assault, around Brussels and served time in at least three prisons.
Mr. Abaaoud and Mr. Abdeslam were convicted on charges of attempting to break into a parking garage in December 2010. Both men served brief prison terms.
Mr. Abaaoud’s most recent arrest came in June 2011, Mr. Chateau said. That was for assault near a metro station in Molenbeek, the predominantly Muslim neighborhood in Brussels where he grew up a few blocks from Mr. Abdeslam and Mr. Abdeslam’s brother, Brahim, who blew himself up in front of the Comptoir Voltaire in Paris on Friday.
Mr. Abaaoud was convicted in August 2011 and released from prison around the end of 2011, Mr. Chateau said.
The last time Mr. Chateau saw Mr. Abaaoud in 2013, he had grown out his beard and become a more observant Muslim, he said.
“He didn’t show signs of radicalization or that he would be implicated in terrorist acts or that he would fight overseas,” Mr. Chateau added.