http://www.suntimes.com/news/metro/1487502,w-fire-casino-empress-joliet-032009.articleTwo-alarm blaze. No injuries reported so far, which is good. The fire started in a section which was under construction--lots of flammables such as sawdust, paints, solvents and construction adhesives in an area under construction.
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I'm going to make a wild stab here and say there are three possible causes of this fire.
Cause 1: arson for insurance fraud. In this economy I wouldn't be expanding my casino.
Cause 2: spontaneous combustion. The facility probably has oiled wood or varnished wood trims--handrails, mouldings, doors. Linseed oil and some varnishes heat up as they dry, and if someone were to wipe up some oil with a cloth, or apply oil with a cloth, then crumple up the cloth and throw it aside, the heat produced by the oil drying is enough to ignite the rag. Before people knew to watch out for this, it happened a lot. Nowadays, not so much; we know to put oil-soaked rags in water now.
Cause 3: the roof. There's a roofing system called "APP Modified Bitumen" that's applied by heating it with a torch. Highly trained and experienced professional roofers don't set buildings on fire doing this. Guys you pick up in front of Home Depot to swing hammers for $8 per hour are not highly trained and experienced professional roofers. If they're using day labor to build this place and they're using APP roofing--a very common commercial roofing system because if it's installed properly it gives the best performance of any system in its price range--it's possible. A lot of church fires have been caused by this system--parishioners who haven't roofed commercial buildings attempt to install an APP roof because it's less expensive than anything else and end up burning the church down.
If we assume the guy didn't burn the place down for the money, I'm going with spontaneous combustion.