Four men accused of belonging to the cell behind the terror attacks in and around Barcelona last week are appearing at the high court in Madrid.
The judge questioning them is deciding what charges to press over the vehicle attacks that left 15 people dead and more than 100 injured.
Eight other members of the alleged cell are dead – some killed in an explosion, others shot by police.
The last suspect was killed in a vineyard west of Barcelona on Monday.
The suspects were brothers and friends of mostly Moroccan origin. Some were not out of their teens.
Catalan police chief Josep Lluis Trapero said the cell was believed to be 12 strong, planning the attacks for more than six months.
The van-ramming plan, which saw 13 people die on Las Ramblas in Barcelona, is thought to have been carried out after the cell accidentally blew up a suspected bomb-making house in Alcanar, 300km south of the town.
A 22-year-old Moroccan, Younes Abouyaaqoub, is thought to have been the driver of the van that rammed into crowds in Barcelona on Thursday.
He was wearing a fake explosives belt and shouted “Allahu Akbar” (“God is Greatest”) before he was shot dead by police, said reports.
The surviving suspects were moved, under high security, from Barcelona to Madrid for Tuesday’s hearing.
Investigating judge Fernando Andreu is expected to read out the charges against them, likely to span counts of terrorism, murder and possession of arms.
If they agree to testify, they will be questioned on their statements.