The terrorists behind deadly attacks in Spain that killed 14 people were preparing bigger atrocities than those they carried out, police said.
As a fourth arrest was made in connection with the deadly vehicle assaults in Barcelona and Cambrils, authorities revealed that a suspected gas explosion on Wednesday robbed the killers of materials to use in larger-scale operations.
Prime Minister Theresa May said a child with dual British nationality was believed to be among those unaccounted for, after a grandfather made a desperate online plea for information about his young grandson's whereabouts following the van attack in the Catalan capital.
Some 34 nationalities were among almost 130 people wounded in the attacks in Barcelona's popular Las Ramblas shopping area on Thursday and in Cambrils, a seaside town 70 miles to the south west, early on Friday.
Catalan regional police official Josep Lluis Trapero told reporters on Friday that Wednesday's blast at a house in Alcanar, a further 55 miles down the coast, meant the attacks were more "rudimentary" than planned.
Reports from Spain had earlier suggested the terror cell may have been planning an attack using gas canisters.
He said: "We are working on the hypothesis that these attacks were being prepared for a while around this private home in Alcanar.
"We think they were preparing at least one or more attacks in Barcelona.
"The explosion in Alcanar at least avoided some of the material they were counting on to carry out even bigger attacks than the ones that happened.
"Because of that the attack in Barcelona and the one in Cambrils were carried out in a bit more rudimentary way than the one they had initially planned."
He added that one of the five terrorists killed by police during the terror attack in Cambrils may have been the driver of the van which killed 13 people in Barcelona.
Four men are in custody, aged 21, 27, 28 and 34. Three are Moroccan and one Spanish, and police said none of them were previously known to the security services for terror-related reasons.
Meanwhile, the grandfather of seven-year-old Julian Alessandro Cadman made a plea for information about the boy, who became separated from his mother during the attack in Barcelona.
Tony Cadman, whose Facebook profile says he lives in Sydney and is from Gillingham, Dorset, posted a photograph of Julian, writing: "My grandson, Julian Alessandro Cadman is missing. Please like and share."
Speaking later from Chequers, Mrs May said: "The UK stands shoulder to shoulder with Spain in confronting and dealing with the evil of terrorism, and I have offered any assistance we can provide.
"Sadly I must tell you that we do believe that a number of British nationals were caught up in the attack and we are urgently looking into reports of a child believed missing, who is a British dual national."
The attacks in Barcelona and Cambrils, some 70 miles south-west of the city on the Mediterranean coast, took place around eight hours apart on Thursday afternoon and in the early hours of Friday.
In a chilling echo of the London Bridge attack in June, Catalonia president Carles Puigdemont said the five terrorists in the Cambrils car were wearing fake suicide belts when they were stopped.
Police revealed that an axe and knives were also found in the vehicle, with one of the latter used to wound one person in the face before the terrorists were gunned down.
Barcelona, a hugely popular tourist destination, came to a halt at noon on Friday (11am BST) as a minute's silence was observed in the Placa Catalunya, close to the scene of the attack.
Led by King Felipe and PM Mariano Rajoy, the silence was followed by applause for the victims.
Three days of mourning have been declared by the government of Catalonia.
The Queen sent a message of sympathy to King Filipe of Spain, saying: "It is deeply upsetting when innocent people are put at risk in this way when going about their daily lives."
Mossos, Catalonia's police force, said on Friday evening that it was continuing to work to identify the driver of the vehicle in Barcelona.
Spanish police were said to be hunting for four men in connection with the attacks, including Moussa Oukabir, who is suspected of using his brother's documents to rent the van which ploughed through pedestrians.
But a union official for Spain's Civil Guard told the Associated Press that police had confirmed Oukabir was one of five suspected terrorists killed in Cambrils.
His older brother, Driss Oukabir, is reported to be one of those arrested following the attack.
According to French media, police are also looking for a white Kangoo vehicle which is believed to have been rented by the suspects and could have crossed the border into France.
Local authorities said they have identified six of the victims of the Barcelona attack. One is Italian, one is Portuguese, three are Spanish and one is Spanish-Argentine.
The victim of the second attack in Cambrils on Friday morning has been identified as a Spanish woman.