According to reports the incident was caused by a mentally unstable man who dipped church vestments in flammable liquid. As of now it also appears that it was an isolated event. But given the recent increase of anti Catholicism in Spain I would not be surprised to find out that this was part of some coordinated effort; all aimed at discouraging people from attending this summer's upcoming World Youth Day, which will be held in Spain. Luckily no one was injured in the fire and most of the basilica is still intact, but still it is a tragic setback to one of Christendom's greatest architectural marvels. To read the US news report click here. To read the UK's The Guardian's article click here.
To read my other two articles about the decline of Catholicism in Spain, The Rapid Decline of Spain and Spain: Groundzero in the Battle Against Relativism.