First rule of troubleshooting is to ask if any work has been done on the equipment recently. I think that can apply to a fire investigation as well. The combination of chemicals, hot work, faulty tools or cables, poor housekeeping practices, and so on, make a fire much more likely.
With that in mind, I'm betting the cause will be traced to a hastily rigged extension cord that shorted out, a spark from some kind of metal cutting or grinding tool that smoldered before flaring up, or a portable heater that was left on.
Anybody know if gas ran up to the 29th floor of this building?
I hope you're right.