BARCELONA, Spain >> The vehicular attacks that fatally crushed at least 14 people in Spain may have been hatched in a house that the plotters were using as a bomb factory, police said today.

At least four suspects were arrested in connection with what Spanish authorities say appears to be a sophisticated and far-reaching plot that could have been much worse.

The attacks carried out Aug. 17 in Barcelona and hours later in the seaside resort of Cambrils wounded scores, shattering Spain’s blissful summer holiday season. The Islamic State claimed responsibility for the Barcelona assault, which accounted for most of the victims.

It was the worst terrorist attack to hit Spain in 13 years and echoed deadly vehicular assaults claimed by the Islamic State in France, Germany, Britain and elsewhere.

Police investigators said they were working under the assumption that the attack in Barcelona, where a van careened down a crowded pedestrian boulevard, and a second incident in which five men in a car hit people in Cambrils were related.

They also linked both assaults to an explosion the night before in the town of Alcanar, 120 miles southwest of Barcelona, that they now believe was evidence of a factory the plotters had been using to make a truck bomb.

Had they succeeded, the carnage could have been far more devastating.

Authorities had been warning of an impending attack for some time, having raised the terrorism threat alert to its second-highest level in 2015. Since the beginning of this year, Spanish police have arrested at least 20 people suspected of links to the Islamic State militant group, the Jane’s Terrorism and Insurgency Center said.

A spokeswoman for Catalan police identified Moussa Oukabir, 17, as a suspect and possibly the driver of the van that careened down Las Ramblas, a major Barcelona street crowded with tourists, killing 13. The driver fled the scene on foot and is now the focus of a sweeping manhunt.

One of Oukabir’s older brothers was among four people arrested in connection with the attack.

Another police official said Oukabir may have been among five assailants killed in the attack in Cambrils, 60 miles southwest of Barcelona, when Spanish police opened fire on a vehicle after it plowed into a crowd, killing one woman.

Investigators said they were working under the assumption that both attacks stemmed from an explosion late Wednesday at a residence in Alcanar that they had initially discounted as a gas accident.

A European counterterrorism expert, who was briefed on the details of the investigation, said police believed the assailants had been manufacturing a sizable device that they aimed to pack into a large truck.

Friday morning, Las Ramblas in Barcelona was thronged again, almost as if nothing had happened. But the mood was subdued, with few customers in the shops.

At the spot where the van driven by the assailant had halted, and where many people were killed or injured, was a pavement mosaic by Joan Miró, the city’s most famous modern artist. In the center of the mosaic lay a makeshift memorial to the victims that included flowers, candles and notes, with one reading, “Barcelona weeps but does not surrender.”

The victims came from at least 34 countries, Spanish authorities said, highlighting how the assailants chose to target one of Europe’s busiest tourist centers at the height of the summer season. Those killed included a 74-year-old Portuguese woman walking with her granddaughter, two Italians and an American. A German official said that several citizens were “fighting for their lives” in the hospital.

Just before a moment of silence for the victims at noon Friday, church bells rang out and people began to move toward the Plaza de Cataluña, the central square of Barcelona. At another makeshift shrine there, two women, one wearing a hijab and the other not, were weeping and holding each other. People nearby chanted in the Catalan language, “The people together will not be beaten,” and, “We are not afraid.”

Political tensions erupted into the open when Catalans remonstrated loudly, with a man with a Spanish flag wrapped around his legs shouting, “This is not the place!” Moments later, opposing groups chanted at each other in Spanish and Catalan.

Others were more reflective, consumed with sadness and worried that the plague of vehicular attacks in Britain, Germany and France over the past two years has now reached Spain.

“I am anxious, nervous, my chest is tight, but at the same time people are going out,” said Estella Gil, a teacher’s assistant, who had come to Las Ramblas in a show of solidarity. “Usually, I walk here feeling safe. Now, I am afraid, really afraid.”

Moussa Oukabir’s brother Driss Oukabir was arrested Thursday after going to police and claiming that his identity documents had been stolen. A national police official, speaking on the condition of anonymity to discuss a continuing investigation, said at least three vans had been rented under Driss Oukabir’s name. He has denied any connection to the attacks.

Two years ago, a person using the name Moussa Oukabir wrote in an online forum that if he were king, he would “kill all infidels and only spare Muslims who follow the religion.”

A third suspect detained in Ripoll, about 65 miles north of Barcelona, has not been identified. It was not clear if his arrest was in connection with the attack in Barcelona, the second assault in Cambrils, or both.

As the scope of the attacks began to emerge, perhaps the most troubling aspect was the apparent existence of a terrorist cell that coordinated the assailants’ actions which might, but for an accident while mixing chemicals for explosives, have been far more deadly.

That blast, which killed one person and injured several others, occurred just before midnight in the town of Alcanar, and was first reported as a gas explosion. But as attacks unfolded in Barcelona and Cambrils later Thursday, police soon made the connection to Alcanar.

The attack in Cambrils was halted by police only after the driver of a compact Audi A3 rammed his vehicle into a group of pedestrians, according to news accounts citing Catalan authorities. Seven people were wounded, and one woman later died of her injuries.

The Spanish newspaper La Vanguardia reported that the Audi had run through a security checkpoint at the entrance to the town, prompting a police chase. After driving into pedestrians, the five occupants emerged, wielding knives. But police quickly descended, killing all five before they could commit any further mayhem.

The five assailants appeared to be wearing explosive vests, although Catalan police said Friday that the explosives were fake. The Islamic State, also known as ISIS and ISIL, claimed responsibility for the attack in Barcelona, but there has been no such claim as yet for the events in Cambrils.

At least 80 people from 34 countries were injured in the assaults. Jean-Yves Le Drian, the French foreign minister, said in a statement that 26 French were among the wounded, with 11 in serious condition. He also said that he would travel to Barcelona on Friday to “visit the French victims of this cowardly act and show France’s support to the Spanish people and authorities.”

The alleged involvement of the Oukabir brothers and their connections to Morocco will no doubt be a focus for authorities: Several of the assailants involved in major terrorist attacks in Paris and Brussels were of Moroccan descent or had relatives there.

The Spaniard was detained in Alcanar, and police were investigating whether he was connected to the explosion Wednesday night.

The attacks Thursday were the most recent in a series of assaults in Europe claimed by Islamic extremists, in which assailants used vehicles to kill people in countries fighting the Islamic State.

Chancellor Angela Merkel of Germany expressed her condolences to the Spanish government and “close solidarity” with the people of Spain “in these difficult hours,” her spokesman said.

Police said that access to the area around Las Ramblas and Plaza de Cataluña would be restricted to pedestrians and that security checks would be conducted, and they warned people not to wear large backpacks or carry large bags.

Catalan authorities have asked the public to come forward with any information about the attack.