The state fire marshal shall appoint one assistant director and such other investigators and employees as the needs of the office require within the limits of the appropriation made for such purpose. Supervising investigators shall be at least twenty-five years of age and shall have either a minimum of five years’ experience in fire risk inspection, prevention, or investigation work, or a degree in fire protection engineering from a recognized college or university of engineering. No person shall be appointed as an investigator or other employee who has been convicted of a felony or other crime involving moral turpitude. Any person appointed as an investigator shall be of good character, shall be a citizen of the United States, shall have been a taxpaying resident of this state for at least three years immediately preceding his appointment, and shall be a graduate of an accredited four-year high school or, in lieu thereof, shall have obtained a certificate of equivalency from the state department of elementary and secondary education, and shall possess ordinary physical strength and be able to pass such physical and mental examinations as the state fire marshal may prescribe. An investigator or employee shall not hold any other commission or office, elective or appointive, or accept any other employment while he is an investigator or employee. An investigator or employee shall not accept any compensation, reward, or gift other than his regular salary and expenses for the performance of his official duties.