Police have raided the flat of an imam who they believe may have radicalised the Barcelona and Cambrils attackers.
It is reported that Abdelbaki es Satty is the ringleader of the terror cell and was responsible for directing the terrorists to commit the attacks in Spain which killed 14 people.
Mr es Satty was reportedly last seen on Tuesday and is said to be about 40 years old - but one line of inquiry being pursued by police is that he may have been killed in an explosion in Alcanar on Wednesday night.
His flatmate told newspaper La Vanguardia that Mr es Satty left his home on Tuesday because he was going to Morocco.
He said he did not hear from him again.
A search warrant seen by Reuters confirms the flat which was raided by police belonged to a man named Abdelbaki es Satty.
He started teaching in mosques in his local area of Ripoll in 2015 and was part of the local Muslim community, but his neighbours told El Pais his teachings about Islam were not radical.
They said he taught Moroccan-dialect Arabic to children and was very reserved.
Pictures have emerged of the inside of Mr es Satty's flat, which show scraps of paper covered in notes strewn around, a picture of a mosque on the wall and a mattress on the living room floor.
Speaking anonymously, a police official said investigators believed Mr es Satty might have been a second fatal victim of a botched bomb-making operation in a house south of Barcelona on Wednesday.
Emergency services had already said that one person died and another was injured in a blast that destroyed the building.
Police believe the house in Alcanar was being used as a makeshift bomb factory to plan the attacks in Barcelona, possibly using large butane gas canisters in even larger-scale attacks.
Head of Catalonia's police, Josep Lluis Trapero, said earlier that the explosion had forced terrorists to scrap their original plan.
"They were preparing one or several attacks in Barcelona and an explosion in Alcanar stopped this as they no longer had the material they needed to commit attacks of an even bigger scope," he said.
The president of the mosque where Mr es Satty preached, Ali Yassine, said he had not seen him since June when he announced he was returning to Morocco for three months.
Ripoll is a quiet town of around 10,000 people in a rural area about 100km (62 miles) north of Barcelona and is a key area linked to almost all the suspects.
Meanwhile, the hunt for the man suspected of driving the van in Barcelona - 22-year-old Younes Abouyaaqoub - continues.
Members of the Spanish royal family were this morning among those who honoured the 14 people killed and 130 injured in the attack.
They attended a special mass at the famous Sagrada Família Basilica in Barcelona.
It has been confirmed this afternoon that missing seven-year-old Australian boy Julian Cadman was among those killed.