Police have found a belt with real explosives at the house where the terror suspects accused of killing 15 people in attacks on Barcelona and Cambrils last week had allegedly been preparing a bigger attack.
Six of the attackers shot dead by police were wearing fake suicide belts.
A police spokesman said on Wednesday that the belt with viable explosives was found at a house in Alcanar, south of Barcelona, where two other cell members died in an explosion last Wednesday prior to the attacks.
Police had already found over 100 tanks of butane gas and materials to make TATP, an explosive used by Islamic State militants, at the house.
Chief investigating magistrate Fernando Andreu provisionally jailed two of the four surviving alleged cell members on Tuesday after hearing testimony in the National Court in Madrid that the group had been preparing massive bombs for an imam who planned to blow himself up at a Barcelona monument.
It was not immediately clear if the belt found amid remains of the blast in Alcanar was meant to be used by the imam, Abdelbaki Es Satty, who allegedly masterminded the attacks and recruited the others in the cell, according to the testimony by the two jailed survivors.
The two - Mohamed Houli Chemlal, 21, and Driss Oukabir, 28 - identified Es Satty as the ideological leader of the cell, according to a Spanish judicial official who attended the proceeding and briefed journalists.
Investigators believe that Houli Chemlal is key to shedding further light on the imam's role and plans as the only survivor in the blast in Alcanar where Es Satty and another cell member died last Wednesday, the official said.
Houli Chemlal was arrested in a hospital after surviving the blast.
He told the court he was alive because he was on the outside porch of the house washing dishes after dinner.
Investigators are also looking into material found during searches in two north-eastern towns, a regional police spokeswoman said.
She declined to say what they found in raids late on Tuesday at a cyber cafe in Ripoll and a house in Vilafranca del Penedes but said the operation was continuing.
Vilafranca is 15 kilometres (nine miles) from where Younes Abouyaaqoub, the suspected driver in the Barcelona attack that killed 13 people, was shot dead by police on Monday.
Abouyaaqoub, who fatally stabbed another person while fleeing on Thursday, is known to have changed clothes, picked up a bag of knives and a fake explosive belt in the time between the Barcelona attack and his death.
The court interrogations on Tuesday exposed more details about how events unfolded during the first hours after the Las Ramblas boulevard carnage on August 17.
That afternoon, another member of the cell abandoned another rented van involved in an accident in a highway near Cambrils, a coastal town south of Barcelona.
According to the police investigation, he regrouped with four other members who bought four knives and an axe in Cambrils three hours before they all were killed by police when they attacked pedestrians in the town's coastal promenade. One woman died in the attack.
Of the four surviving suspects of the cell, Sahl El Karib worked in the Ripoll cafe. Judge Andreu ordered him to be kept in custody for three more days while police continued probes.
Oukabir, El Karib and fourth suspect Mohamed Aalla denied being part of the cell during Tuesday's testimony, said the National Court official who attended proceedings.
Es Satty preached in a mosque in Ripoll, which was home to most of the alleged cell members.
IS has claimed responsibility for both the Barcelona and Cambrils attacks, which also wounded more than 120.