just as you would investigate whether a given state uses a medical examiner vs. coroner, I'd imagine you would want to check for the particular state to see if there are differences there too with regards to arson
most states have state officials called State Fire Marshalls who make that determination...but, it is important to note that they only respond when called by the local fire dept or law enforcement, it's not simply a given that a Fire Marshall attends a fire scene post-loss
if a fire is deemed "suspcious" by law enforcement/fire dept, they'd have the fire marshall there ASAP, within 24 hours or so...but, if the arson suspicions don't arise until much later, let's say in an insurance claim scenario where the items claimed to be lost in the fire don't add up...well, then an insurance company might contact the fire marshall and any fire marshall I know would definitely take a look to see if in fact charges need to be brought criminally for the fire....or, the insurance company might hire private arson experts (cause and origin experts) to make the determination even without the state's involvement