French gendarmerie officers patrol near the Eiffel Tower in Paris on Nov. 23. (Jacques Brinon/AP)

To carry out the attacks that left 130 people dead in Paris this month, the killers relied on a cunning awareness of the weaknesses at the heart of the European security services charged with stopping them.

Poor information-sharing among intelligence agencies, a threadbare system for tracking suspects across open borders and an unmanageably long list of homegrown extremists to monitor all gave the Paris plotters an opening to carry out the deadliest attack on French soil in more than half a century.

Two weeks later, European security experts say the flaws in the continent’s defenses are as conspicuous as ever, with no clear plan for fixing them.

“We lack the most obvious tools to deal with this threat,” said Jean-Charles Brisard, chairman of the Paris-based Center for the Analysis of Terrorism. “We’re blind.”

With the Syrian war raging on the continent’s doorstep and thousands of Europe’s own citizens traveling to and from the battlefield under the influence of a spellbindingly effective propaganda campaign, Brisard’s bleak assessment is widely shared.

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

The mismatch between the scale of the threat and Europe’s patchwork response has contributed to a grim resignation among counterterrorism professionals: Even after a series of terrorist strikes this year — including two bursts of mayhem in Paris, deadly shootings in Copenhagen and a would-be assault on train passengers foiled by off-duty U.S. servicemen — another large-scale attack in Europe is almost inevitable.

“We have to figure out what went wrong and fix it as soon as possible. Because one thing is for sure: Islamic State will try to hit Europe again,” said a senior European intelligence official who spoke on the condition of anonymity.

Unlike in the United States after the Sept. 11, 2001, attacks, when American officials vowed to do whatever it took to prevent a repeat, Europe’s leaders can offer few guarantees. They face enormous structural holes in their security networks, and they have few obvious solutions to a threat more potent than any the continent has confronted in decades.

The Paris attackers freely exploited those flaws and offered a possible guide to others who could follow in their blood-saturated wake.

Coordination between European intelligence services is poor, with no comprehensive, shared list of suspected extremists. So the attackers hopped freely and frequently over unguarded European Union borders, with at least five also traveling to Syria and back.

Most had already been flagged as potential security threats. But so had tens of thousands of others — 20,000 in France alone — and the plotters were careful not to stand out or give law enforcement an excuse to arrest them.

On Friday, Nov. 13, the worst terrorist attacks in Paris since World War II took place. In the days after the attacks, French authorities responded with raids, airstrikes and a manhunt. (Monica Akhtar, Deirdra O'Regan/The Washington Post)

The attackers chose lightly guarded targets, probably conscious that doing so would only add to the burden of security services already buckling under the strain of austerity-imposed budget reductions.

“The systems of European security that at one time were useful and effective are no longer adapted for this threat,” said Bernard Squarcini, a former head of France’s domestic intelligence service who now leads the Paris office of the global intelligence firm Arcanum. “We are dealing with people who are cunning and determined. They’ve been in combat.”

European security officials have warned for more than two years about the threat of citizens returning from the Syrian battlefield to wage war at home. But the Paris attacks revealed how ill-equipped the continent is to reckon with the problem.

Despite returnees being at the top of Europe’s threat list, the attackers seemed to face little trouble shuttling back and forth between Islamic State-controlled territory in Syria and the downtrodden neighborhoods of Brussels and Paris, where preparations for the attacks were finalized.

The failures reflect the paradox at the heart of Europe’s security dilemma: The continent’s citizens can freely cross borders, but authorities lack access to shared databases on suspected terrorists.

One of the attackers, 28-year-old Samy Amimour, was placed under judicial supervision in France in 2012 after attempting to travel to Yemen. But he later managed to get to Syria and back.

Belgian law enforcement was aware that 20-year-old Bilal Hadfi had returned from Syria but couldn’t find him.

Another attacker, 31-year-old Brahim Abdeslam, was caught on his way to Syria by Turkish authorities. Belgian law enforcement questioned him — then let him go. His 26-year-old brother, Salah, was also questioned and released — even though Belgian authorities knew he had become radicalized. Hours after the attack, French police stopped his car. But they let him go, and he remains at large.

Abdelhamid Abaaoud, the alleged architect of the attacks, was one of Europe’s most wanted men before the attacks. But the 28-year-old Belgian slipped from the radar of intelligence services and was widely thought to be in Syria, where he starred in grisly propaganda videos.

In fact, he had returned to Europe. But there was no trace of him until the night of Nov. 13 — when phone records show he stood watching from the street as police battled the militants he had sent to kill young music fans at Paris’s famed Bataclan concert hall.

At the root of the intelligence failures, Brisard said, is a European security system that was designed to guard against external threats and is ill-prepared now that it faces such a sprawling challenge from its own battle-hardened and radicalized citizens.

“The paradigm has changed,” the terrorism analyst said. “We need to adapt.”

A good starting point, he said, would be a systematic way of checking E.U. citizens against security databases when they return from outside the union’s borders. Such checks have been sporadic, as border guards typically confirm that the face of the traveler matches the one in the passport.

After the attacks, the E.U. stepped up its controls. But their effectiveness is severely limited. Europe lacks a common biometric identification system, and the one shared database covers only those with criminal records, not those who are suspected of extremist plots.

“We need a Europe-wide blacklist of jihadists,” said Manfred Weber, the head of a center-right group in the European Parliament that is pushing for tighter controls.

The idea of creating a “Passenger Name Record” — similar to the U.S. no-fly list — has been debated by E.U. politicians and bureaucrats for more than a decade. But it has been repeatedly stalled by privacy concerns among those who say the United States went too far in its response to Sept. 11 and who don’t want to repeat the same mistake.

There is little intelligence-sharing across Europe, despite the continent’s open borders. Intelligence services prefer to cooperate on a bilateral basis with favored partners, rather than distribute information across a 28-member bloc. Even after the attacks, analysts say, that’s unlikely to change at a time when terrorism, migration and debt are pulling E.U. members apart rather than bringing them together.

“The crises facing Europe are taking their toll on trust and unity,” said François Heisbourg, chairman of the International Institute for Strategic Studies.

The strains can be seen in European responses to the attacks, which have been marked by unanimous expressions of solidarity with France — but also little cohesive action and occasional finger-pointing.

French officials have accused Belgium of allowing Islamic extremism to incubate unchecked. Belgian authorities have hit back, but some have also admitted that the country is failing to meet the challenge.

Belgian security forces suffer from tensions between their French and Flemish halves. Hard-line Saudi preachers have long been allowed to preach in the country’s mosques, and a thriving black market for arms has made weapons readily available.

More Belgians per capita have traveled to Syria to fight than from any other E.U. country. Relatives of those who have joined the tide blame their government and security services for a lack of oversight and prevention.

The night she realized her 22-year-old son had left for Syria, Yasmine called police to inform them who had recruited him: Jean-Louis Denis, a Belgian convert.

But Yasmine, who spoke on the condition her last name not be used in order to protect her family, said authorities failed to act.

Her younger son, 16, followed his brother to Syria three months later, in April 2013. Denis was not arrested until more than a year and a half after that, despite being monitored by authorities since 2009. All the while, he was sending young Belgians to Syria — a crime for which he was ultimately convicted.

“They know what’s happening, but they don’t intervene,” Yasmine said.

Alain Winants, head of Belgium’s domestic intelligence agency until 2014, said the service simply could not cope with the strain of so many citizens becoming radicalized so quickly. He was lobbying for a 20 percent increase in personnel when he left. The agency has 600 employees.

France has considerably more agents in its domestic security service — about 3,300. But they are tasked with monitoring 20,000 people on national security watch lists, about half of whom are said to be Islamist extremists.

Experts say it is impossible for security services to run surveillance on such a large number. But unless the suspects commit a crime, they can’t be arrested, either.

Following the Paris attacks, French President François Hollande declared a state of emergency that has given security services vast new powers. Authorities have conducted more than 1,000 searches, more than 120 people have been charged, and many others have been placed under house arrest.

The measures, which will remain in place for three months, may disrupt plots in the short term. But, given the civil liberties concerns, Heisbourg said, they are hardly long-term solutions for Europe’s struggle with violent extremism.

“My house could be raided by cops tomorrow without a warrant. This is very radical stuff,” he said. “We’re doing the right thing. But this can’t be made into the new normal.”

Morris reported from Brussels. Virgile Demoustier in Paris, Karla Adam in London and Souad Mekhennet contributed to this report.