One of Mr. Oukabir’s older brothers was among the four people arrested in connection with the attack.
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 the 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, the 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 out 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, 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 across Europe over the past two years had 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.”