Mont. Code Ann. § 46-30-301

The arrest of a person may also be lawfully made 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 of 1 year or more. When arrested under this section, the accused must be taken before a judge or magistrate with all practicable speed and complaint must be made against the accused under oath setting forth the ground for the arrest as provided in 46-30-227. After the complaint is made, the accused's answer must be heard as if the accused had been arrested on a warrant.