Arson is a criminal act, and insurance companies write their policies to exclude deliberately set fires. Otherwise, fraudsters would be able to collect insurance despite breaking the law.
Local, state, and sometimes even federal government agencies may participate in an arson investigation. First responders to a fire may relay unusual things they’ve seen to investigators, whose job it is to document the scene for strange burn patterns, the presence of accelerants, and tampered utilities.
Fire insurance policies will not cover fires that are set on purpose, only accidental losses.