In your estimation why would the FIB ...errrr, FBI...be so quick to rule out terrorism?
It actually happens a lot these days [that is ruling it accidental quickly], but usually it's ATF that performs the investigation not the FBI. I worked on a fire last week that was ruled accidental by ATF, but myself and two other investigators categorized it as undetermined until we complete our investigation. Our opinions as to where the fire originated were substantially different than those of ATF's.
Why rule them accidental so quickly? I don't know. Maybe to keep it out of the spotlight as much as possible until they can collect the data. Maybe to keep unecessary speculation from occurring and creating panic.
The appropriate category though would be undetermined [which means it could be accidental or criminal - in other words - we don't know YET].
I know some good ATF fire investigators. So I'm not pounding them. Maybe it's political. I also noted that the Chemical Safety Board is investigating. I've worked with those guys and they are very professional and good at getting to the root cause.
A refinery does contain the ingredients for a catastrophic explosion - accidental or otherwise. Just have to wait for all the data to be collected and not jump to conclusions.