BARCELONA • When an earthshaking explosion on Wednesday blew apart a house outside Alcanar, a town surrounded by olive groves and holiday homes overlooking the Mediterranean, the police first blamed it on a gas leak.

"Nothing ever happens here," Mayor Alfons Monserrat said.

The Spanish police now believe that tiny Alcanar may have been the incubator for a plot far more ambitious than the vehicular attacks in Barcelona and Cambrils that killed 14 people and injured scores.

It was Spain's worst terror attack in more than a decade, and the Islamic State in Iraq and Syria terror group has claimed responsibility.

The Alcanar blast, the police suspect, was the result of a mistake by the plotters, who had intended to make a powerful bomb, place it in a van and detonate it in the crowded centre of Barcelona.

That plan disintegrated along with at least 12 butane gas canisters that were discovered in the ruins of the house on Wednesday night.

Four men have been detained in the case, and three more remain at large, according to Major Josep Lluis Trapero, a senior police official in Spain's Catalonia region.

Five suspects are dead, at least three of them appearing to be so young that they could not have grown beards. They were killed by the police during a second attack in the seaside town of Cambrils early on Friday.

While some of the other recent European terror attacks have been opportunistic hit-and-runs by individuals acting on their own, this was a comparatively complicated plot that the police say involved at least two cells working in several different locations across Catalonia.

The story also unfolded in Ripoll, home town of one of the young men, Moussa Oukabir, 17, who was killed in Cambrils. His brother Driss, 28, was arrested after his identity documents were found to have been used to rent the van used to carry out the attack on Las Ramblas, Barcelona. At least one other person from Ripoll has been detained.

There were few signs the brothers had become radicalised. Ripoll is a mountain town north-west of Barcelona of about 10,000 people, and the two Oukabir brothers, both of Moroccan descent, lived there with their mother and sisters.

Among neighbours, friends and former employers, no one saw any outward sign of budding extremism. Driss spoke perfect Catalan as well as Spanish and was not religious, according to a childhood friend, Raimon Sanchez, 27. He was known as a small-time marijuana dealer, but nothing more.

"We went to school together. After that, everyone went his own direction. But when we saw each other we would say hello," Mr Sanchez said. "He was in my home when he was a child - how can a person change that much?"

Moussa was well liked by everyone. He also spoke perfect Catalan, said a neighbour. His two sisters described him to their former employer at a local restaurant as polite, "having really good marks in school" and eager to study.

On Thursday afternoon, one of the attackers arrived at a branch of Telefurgo, a car rental firm, about 24km north of Barcelona, where he used the identity documents of Driss to hire a white Fiat Talento, the company told Spanish journalists.

At around 5pm, that white van was zigzagging down Las Ramblas, mowing down people in its path.

About eight hours later, another car attack unfolded in Cambrils. All five assailants in an Audi A3 were shot dead, including Moussa.

Twenty-four hours after the Alcanar blast, the police believe that the town may have been where the attacks were planned - news that shocked local residents.

"You might think all sorts of things," said Ms Nuria Gil, 50, one of the residents, "but not that you have terrorists as neighbours."

NYTIMES