The Barcelona terror gang planned to murder thousands of people using an explosive known as the Mother of Satan.
It is thought they intended to simultaneously detonate bombs in vans at three locations in the Spanish city.
Targets included the Sagrada Familia church, a Unesco World Heritage Site, which is one of the most visited attractions in Europe.
The other two locations were the port area of the city and the Ramblas, where a van was deliberately driven into crowds on Thursday afternoon, killing 13 and injuring 130.
