Mo. Rev. Stat. § 546.620: Power of sheriff to summon aid.

Such sheriff or deputy, while conveying a convict to the penitentiary, shall have the same power and like authority to require the assistance of any citizen of this state in securing such convict, and retaking him if he shall escape, as if such sheriff or deputy were in the county in which he is such officer; and all persons who shall neglect or refuse to assist such sheriff or deputy, when required, shall be liable to the same penalties as if such officer were in his own county.