Cathedral Twitter page with great photos. This was a horrible, and hopeful, accident and I hope not a deliberate terrorist attack. Given the location of where it seemed to start and what was going on I believe it's very reasonable to assume it was a horrible accident or potentially some rags or chemicals spontaneously erupting. Either way a horrible loss but it does appear that there are some areas that miraculously escaped the fires rage. Given the age of the wood in the place it's a miracle that any of it was saved. Fox News is now saying that arson and terror has been ruled out which, although it doesn't make it much better, does not make it worse.
It also appears that many irreplaceable carvings may have been saved from the flames. Even within the Sanctuary the cross is highly visible and appears that much of the inside at least will be able to be salvaged and potentially used when they rebuild.