Mo. Rev. Stat. § 569.040: Arson in the first degree — penalty.

1. A person commits the offense of arson in the first degree if he or she:

(1) Knowingly damages a building or inhabitable structure, and when any person is then present or in near proximity thereto, by starting a fire or causing an explosion and thereby recklessly places such person in danger of death or serious physical injury; or

(2) By starting a fire or explosion, damages a building or inhabitable structure in an attempt to produce methamphetamine.

2. The offense of arson in the first degree is a class B felony unless a person has suffered serious physical injury or has died as a result of the fire or explosion set by the person or as a result of a fire or explosion started in an attempt by the person to produce methamphetamine, in which case arson in the first degree is a class A felony.