Mo. Rev. Stat. § 9.135

The United States flag and the Missouri state flag shall be flown at half-staff on all government buildings for one full day whenever a Missouri resident, who was a resident at the time of enlistment in military service or at time of death, is killed in the line of duty during service in the military forces of this state or the United States. The Missouri veterans commission shall make ongoing reasonable efforts to determine if any residents have been killed in the line of duty, and shall notify the governor of any such death. The governor, who shall determine the day on which the resident shall be honored, shall then notify the office of administration.