Mo. Rev. Stat. § 548.141: Arrest without a warrant.

The arrest of a person may be lawfully made also by any peace officer or a private person, without a warrant upon reasonable information that the accused stands charged in the courts of a state with a crime punishable by death or imprisonment for a term exceeding one year, but when so arrested the accused must be taken before a judge or associate circuit judge with all practicable speed and complaint must be made against him under oath setting forth the ground for the arrest as in section 548.131; and thereafter his answer shall be heard as if he had been arrested on a warrant.