Spanish authorities believe there may have been eight people involved in a cell which carried out an attack in Barcelona.
A judicial source with knowledge of the investigation also said the group had planned to use butane gas canisters.
Catalan government official Joaquim Forn told local radio earlier on Friday that it was possible that attackers had meant to use canisters in the attack on Thursday in which a suspect drove a van at speed along a busy pedestrian street.
Security forces are hunting for the van's driver, who was seen escaping on foot, and police said they had killed five attackers on Friday night in Cambrils, a town south of Barcelona, to thwart a separate attack.
"The priority right now is work out the identity of these people, to prove and show the relationship between the different people involved, those that took the van and those that have been able to escape," Forn said.
