Officers carried out new raids Wednesday to root out a possible support network for the men, accused of ploughing vehicles into pedestrians.
Officers carried out new raids Wednesday to root out a possible support network for the men, accused of ploughing vehicles into pedestrians on Barcelona's busy Las Ramblas boulevard last Thursday and a seaside promenade in the resort town of Cambrils just hours later, killing 15 including a young boy and wounding more than 120 people.
Barcelona's mayor, Ada Colau, and the head of the regional government Carles Puigdemont were due to participate in an interfaith ceremony Thursday in memory of the victims.
The international connections of the cell of mostly Moroccan nationals have also come under scrutiny as investigators retrace their movements to France and Belgium.
The scale of the assaults being prepared by the jihadist suspects emerged during a preliminary court hearing Tuesday, when Mohamed Houli Chemlal, 21, told the judge the group was planning "an attack on an even greater scale, targeting monuments" using bombs.
After Chemlal's chilling admission, Barcelona authorities said security was being boosted at key tourist sites including the iconic Sangrada Familia church as well as at major events.
At least 500 litres of acetone, large quantities of nails and detonators as well as gas canisters have been found at a house in the town of Alcanar south of Barcelona, court documents said.
They are ingredients of TATP -- the explosive of choice of the Islamic State group, which has claimed its "soldiers" carried out the attacks.
But an accidental blast at the bomb factory in Alcanar on August 16, the eve of the Barcelona van attack, forced the cell to alter its plans and turn to vehicles as killing machines.
After losing their bombs and two cell members in the blast that was so powerful that "the mushroom cloud that was generated was visible several kilometres around", the jihadists put plan B into action.
They rented a van the next day, but again suffered a false start as it got into an accident not far from Cambrils.
A second van was used two hours later in the Barcelona rampage.
After a full day court hearing, Chemlal and another suspect, Driss Oukabir, 28, were Tuesday remanded in custody and charged with terror-related offences.
A third man, Mohamed Aallaa, 27, who owns the car used in the Cambrils attack, was granted conditional release as the judge deemed evidence against him weak.
The judge delayed a decision on the fourth suspect, Salh El Karib, whose shop allows people to make calls abroad and was among targets of police raids late Tuesday.
The men in court Tuesday were the only surviving suspected members of the terror cell, which Spanish police said they had dismantled after gunning down the last man at large and the Barcelona van driver, Younes Abouyaaqoub, on Monday.
Chemlal was injured in the Alcanar explosion that killed an imam, Abdelbaki Es Satty, who is thought to have radicalised him and other young suspects. He said the imam had wanted to blow himself up.
A Spanish judicial body said Wednesday the imam was due to be deported after serving four years in jail. But a court annulled the decision in March 2015, finding that he was showing efforts to integrate in Spain.
In the rubble of the Alcanar house, police found a sheet of paper slipped into a green-coloured book, which said: "A brief letter from the soldiers of the Islamic State on the territory of Al Andalous to the crusaders, the sinners, the unjust and the corrupters".
Al Andalous is the name of the territories in modern Spain ruled until 1492 by Muslims.
Meanwhile in Granada, southern Spain, 200 Muslims rallied in the streets on Wednesday to protest against a surge in anti-Islamic hate crimes following the attacks.
Focus is turning to the cell's links abroad.
Spanish press reported arrests in Morocco in relation to the case, but Rabat declined comment when contacted by AFP.
In Rotterdam, Dutch police were investigating possible terror links after Wednesday arresting a Spaniard driving a van containing gas canisters close to a rock concert which was abruptly cancelled over fears of an attack.
In Belgium, the mayor of the Vilvorde region told AFP that Satty spent time in the Brussels suburb of Machelen -- next to the city's airport -- between January and March 2016. He tried to get a job in a local mosque but was refused due to the violence of his sermons.
And French Interior Minister Gerard Collomb said the Audi used in Cambrils had been detected speeding in the Paris region while making "a very rapid return trip" days before the Spanish attacks.