Those are bad news, really
I've looked a little the news about the Sagrada Familia, and that's what I can tell you: an insane man, with pockets full of lighters, went to the sacristy of the temple, a place with a lot of imflammable material (the wardrobes with the clothes of the priestess, for instance), and set fire. Some tourists that were visiting the building caught him.
The damage was important, but it could be worse. It seems that the sacrist is all burnt, but no original Gaudí work has been affected. The firemen had to enter somewhere, and they were almost forced to enter through the stained glass windows, but VERY LUCKILY they finally were able to enter through the door. The crypt suffered minor damaged because of smoke.
The temple re-opened to visitors the same day later, except, of course, the sacrist.
It was bad, but it could be much worse