PARIS – Although not as deadly as they might have been, the terrorist attacks in Barcelona on Thursday afternoon and at a Spanish beach resort in the early hours of Friday appear to be the most extensive coordinated operations claimed by the so-called Islamic State since the carnage in Paris in November 2015 and Brussels in March 2016.

A driver in a panel truck racing down Las Ramblas, in the heart of the Barcelona tourist district, mowed down strollers and café goers from dozens of different nations shortly after 5 p.m. on Thursday, killing 14 people and injuring more than 100.

Eight hours later, an Audi A3 sedan with five men crammed inside attempted to carry out a similar operation on the seaside walkway of Cambrils, but it crashed and appears to have turned over as police converged on it. Some of the men reportedly emerged from the car wielding knives, but four were shot dead at the scene and one, seriously wounded, died in hospital. A handful of police and bystanders were injured, but none killed.

Spanish authorities concluded belatedly that an explosion Wednesday which brought down part of a building in the town of Alcanar, still farther down the Catalan coast, was the result of bomb-making gone awry. The Catalan police initially thought the blast was an accident, possibly related to drug making. One person there was killed, and one who was injured is reportedly now under arrest.

Another man, Driss Oukabir, was arrested when his documents were linked to the murder vehicle on the Ramblas. But after his photograph was widely disseminated by police, he turned himself in to authorities in the town of Ripoli about 60 miles north of Barcelona, claiming his passport had been stolen. His 18-year-old brother, Moussa, is now a prime suspect in the attack there and apparently still at large.

In all, four people are now reported under arrest.

There is growing speculation among terrorism experts that the Alcanar blast forced a cell of at least eight people to act much sooner than they had planned. (Something similar happened in Belgium when a high profile arrest led the plotters there, who had planned to attack European soccer championships in France, to rush instead to blow themselves up at the Brussels airport and in the Brussels subway.)

But a cell of eight or more jihadis operating in Europe suggests an important, indeed fatal failure of intelligence.

So-called “lone wolf,” or at least lone attacker, operations are very difficult to penetrate, but once the network expands police and intelligence services like to think that their informers and their monitoring of communications will enable them to intercept and disrupt terrorist plots.

Note that other vehicle attacks in Nice on Bastille Day last year (86 dead), at a Berlin Christmas market in December (12 dead), London’s Westminster Bridge in March (six dead including the attacker), Stockholm in April (five dead) were carried out by individuals who may have had a support network and direct links to ISIS, as in the Berlin case, or have acted mainly on their own, as in Nice.

The Manchester Arena attack—at an Ariana Grande concert full of young girls—killed 23 people in May, and was carried out by a lone suicide bomber, although he had extensive contacts with other jihadis, including family members.

But the London Bridge and Borough Market attack in June killed eight people, plus three terrorists “neutralized” at the scene, suggesting a revival of multi-attacker operations, and it may have been a model for attack that just took place in Spain.

Why wasn’t the cell in Catalonia uncovered quickly after the explosion in Alcananar?

Some reasons cited by locals and outsiders alike relate to the proudly and sometimes vehemently observed autonomy of Catalonia, famous for its capital Barcelona, its beaches along the Costa Brava, and its own language, Catalan, used for official business.

Gilles Kepel, a French authority on jihadi networks and author most recently of Terror in France notes that since the horrific attacks of 2015 in Paris, jihadis have come under massive pressure from the unified French police and intelligence services, which may explain why they have stepped up operations elsewhere in Europe.

“Britain, Germany, and Spain to a large extent have decentralized intelligence and police communities, as opposed to France, which has a more centralized system,” says Kepel.

But Catalonia is a particularly problematic case. In the past, various Catalan nationalist politicians preferred to import North African labor (even though they sometimes referred to them as “the Moors”) rather than those from elsewhere in Spain (los inmigrantes).

More recently, on such a basic matter as fundamental defensive measures adopted by many European cities after Nice and London—the placing of obstacles along wide pedestrian thoroughfares like Las Ramblas—Catalan authorities allegedly wanted to show they would take an approach different from Madrid. So, no bollards to stop a vehicle from blasting through pedestrians.

Intelligence sharing with the central government may also have been affected, making it more difficult to tie together threads that stretch across regional borders, let alone international ones.

Analysts at Jane’s Terrorism and Insurgency Centre (JTIC) database are among those who point to the potential involvement of a wider network and more coordination than in other recent terror incidents across Europe. They also point to the many warning signs that something was brewing in Catalonia.

According to the JTIC, Spanish police have arrested at least 20 suspects connected to the Islamic State. “Notably, 11 of the suspects detained in 2017 have been arrested in Catalonia, where the latest attack occurred,” the JTIC reports. “Of 38 counter-terrorism operations conducted in 2015 and 2016, 10 operations leading to the arrests of 24 suspected Islamist militants were conducted in Catalonia. Since the start of 2015, 43.2 percent of arrests targeting Islamist militants recorded by JTIC have taken place in Catalonia, highlighting it as a hub of Islamist activity in Spain.”

Yet, for all that, “the two attacks and the relatively large geographic dispersal between Barcelona and Cambrils, the involvement of a larger number of people [than in other attacks in Europe in 2016 and 2017] and the potential discovery of a site to prepare powerful explosives in Alcanar suggest a much higher level of coordination than has been typically present in previous attacks.”

Spain has been on its next-highest level of alert since 2015, in the aftermath of the Charlie Hebdo and kosher supermarket attacks early that year.

But the autonomous Catalan government has been pressing forward with plans for a highly contentious referendum on complete independence scheduled for October 1, and its secessionist sentiment, though by no means universal, may have had particular relevance to these attacks.

Hugo Micheron, a researcher who has interviewed scores of jihadis in French prisons and elsewhere, notes that those operating in Europe often see elections as key moments for their operations. “ISIS wants to destabilize the democratic process,” he says.

In 2004 in Madrid, terrorists linked to al Qaeda struck the main commuter railway station days before voters went to the polls to choose a new government, killing 192 people and injuring 2,000. More recently, the London Bridge attack came five days before Britain’s snap parliamentary elections in June. And before the French elections in April, although only one terror operation took place, a shooting that killed a policeman on the Champs-Élysées, several major attacks were disrupted.

Minutes after the attack in Barcelona, authorities activated a national terrorist emergency plan that is supposed to pull together national, autonomous regional, and local security forces in what was called Operación Jaula, Operation Lock-Up, and there is no doubt that the Catalan police performed heroically confronting the attackers in Cambrils.

But conflicting reports suggest that in the immediate aftermath of the explosion in Alcanar, where multiple propane gas canisters were discovered, the Catalan police refused the assistance of TEDAX, a unit of the Spanish government with long experience dismantling bombs and investigating explosive evidence dating back through decades of Basque separatist terrorism. If true, valuable hours may have been lost as the killers raced to go into action.

As of this writing, the precise sequence of events after the attack at Las Ramblas were not entirely clear.

Initially there were reports that the driver of the van was carrying a rifle or shotgun and may have taken hostages in a restaurant. Those proved untrue. But the driver did escape on foot in the immediate aftermath of his killing spree when the car came to a stop near a famous mosaic by Joan Miró.

There were also reports of a second van, found miles away from the scene near a Burger King, but what role if any it played in the operation is unclear.

Most mysterious of all was an incident at 7:24 p.m., two hours after the Ramblas attack and after Operation Lock-Up threw up a cordon of checkpoints around the city. A Ford Focus rammed through a police checkpoint, injuring a woman officer. Her partner opened fire, emptying his magazine into the car as it drove away. Several kilometers farther down the road, police found the vehicle with a body slumped inside. But it wasn’t in the driver’s seat, and it didn’t show any sign of bullet wounds. Instead it appeared to have been stabbed several times with a knife, leaning forward from the backseat over the gearshift.

Current speculation among police reporters in Barcelona is that the Ford was indeed used as a getaway car by the Ramblas driver, perhaps after it was hijacked and the owner stabbed to death.