Mo. Rev. Stat. § 540.140: Rights and privileges of prosecuting or circuit attorney.

The prosecuting or circuit attorney shall be allowed at all times to appear before the grand jury on his request, for the purpose of giving information relative to any matter cognizable by them, and shall be permitted to interrogate witnesses before them, when they or he shall deem it necessary. No prosecuting or circuit attorney or any other officer or person, except the grand jurors, shall be permitted to be present during the expression of their opinions or the giving of* their votes on any matter before them.