Police confirmed on Friday afternoon that the suspects had been planning to detonate explosives.
Spain's King Felipe VI and Queen Letizia defiantly joined almost 1,000 mourners Sunday at the stunning cathedral, as word spread that the house of worship had been the terrorists' initial target, according to the Spanish newspaper El Español.
As investigators scrambled to piece together the attacks which killed 14 people in all, Interior Minister Juan Ignacio Zoido said yesterday the cell behind the carnage that also injured 120 and plunged the country into shock had been "dismantled", though local authorities took a more cautious tone.
Two days after the assaults that struck the busy tourist hub and the nearby seaside town of Cambrils, Spaniards put on a defiant front while mourning the victims, with crowds out in force to greet King Felipe and Queen Letizia as they arrived to pay homage to the victims.
Younes Abouyaaqoub, a 22-year-old Moroccan, is suspected of driving the van used in the attack on Las Ramblas. The third was found in Ripoll itself. A 2004 terror attack in Madrid that killed 192 was carried out by an al-Qaida cell.
Police now believe that due to the unexpected explosion in Alcanar, the terrorists had to resort to "more rudimentary" vehicle attacks in both Barcelona and Cambrils.
"Es Satty's apartment mate told NPR this morning that the imam cleaned out his bedroom here last Tuesday", a day before the house explosion, Langfitt reports for Weekend Edition Sunday.
Suspects Moussa Oukabir, Mohamed Hychami and Said Aallaa are believed to have been killed.
On investigation, police believe the explosion was caused by the mishandling of explosives intended for a multiple - and much larger - terrorist attack on Barcelona.
The victims of the terrorist attacks in Catalonia are of 34 nationalities, the Catalan emergencies service said Friday. His older brother, Driss, counts among the four arrested. The royal couple is expected to visit the place of the attack on Barcelona's central La Rambla pedestrian mall later on Saturday.
He reportedly died along with Said Aallaa, 19, and Mohamed Hychami, 24, who were part of a group that mounted a similar attack in Cambrils that left one woman dead and six people injured.
"We don't have any specific information on this but it can not be ruled out", he said, adding that border checks had been reinforced immediately after the attacks.