I'm going to be an idiot and call it right here: a welding or cutting torch. Why is it always some unlucky fool with a sparky-sparky machine? Yeah, I know, sparks and all that, but it and the fire triangle are not exactly new technologies and we know how to handle everything but a flaming comet or a fire tornado, especially for a treasured national monument. Automated thermal imaging and sensor systems with multiple backups, on-site water pumps with misters and fire training, fire extinguishers, site preparation, spotters, no work on windy days and all that...*sigh*
But they probably already had all that and someone was just careless or unlucky or "we never thought this could happen" happened or "someone used A instead of B".
Damn.