On Aug. 17, 2017, Spain suffers two terrorist attacks perpetrated by young people integrated into Spanish society. How could something like this happen?

On August 17, 2017, in Barcelona, a van hurtled at top speed across the 800 meters separating the Plaza de Cataluña from the Joan Miró mosaic, smashing into a crowd of people. Hours later, another attack struck Cambrils, in Tarragona. These terrorist attacks were perpetrated by young men who were believed to be integrated into Spanish society. How could something like this happen?