The fire investigators were probably following NFPA 921. 921 states:

19.2.1 Classification of the Cause. Classification of a fire cause may be used
for assignment of responsibility (see Section 18.6), reporting purposes, or
compilation of statistics. Different jurisdictions may have alternative
definitions that should be applied as required. The cause of a fire may be
classified as accidental, natural, incendiary, or undetermined. Use of the term
“suspicious” is not an accurate description of a fire cause. Suspicion refers to a level of proof, or level of certainty, and is not a classification for a fire cause. Suspicion is not an acceptable level of proof for making a determination of cause within the scope of this guide and should be avoided. Fires in which the level of certainty is probable or suspected, or in which there is only suspicion of that cause, should be classified as undetermined.