104 and in national legislation, warrant the conclusion that case law reflects customary rules of international criminal law. 227. In sum, the objective elements (actus reus) of this mode of participation in one of the crimes provided for in the Statute (with regard to each of the three categories of cases) are as follows: i. A plurality of persons. They need not be organised in a military, political or administrative structure, as is clearly shown by the Essen Lynching and the Kurt Goebell cases. ii. The existence of a common plan, design or purpose which amounts to or involves the commission of a crime provided for in the Statute. There is no necessity for this plan, design or purpose to have been previously arranged or formulated. The common plan or purpose may materialise extemporaneously and be inferred from the fact that a plurality of persons acts in unison to put into effect a joint criminal enterprise. iii. Participation of the accused in the common design involving the perpetration of one of the crimes provided for in the Statute. This participation need not involve commission of a specific crime under one of those provisions (for example, murder, extermination, torture, rape, etc.), but may take the form of assistance in, or contribution to, the execution of the common plan or purpose. 228. By contrast, the mens rea element differs according to the category of common design under consideration. With regard to the first category, what is required is the intent to perpetrate a certain crime (this being the shared intent on the part of all co-perpetrators). With regard to the second category (which, as noted above, is really a variant of the first), personal knowledge of the system of ill-treatment is required (whether proved by express testimony or a matter of reasonable inference from the accused’s position of authority), as well as the intent to further this common concerted system of ill-treatment. With regard to the third category, what is required is the intention to participate in and further the criminal activity or the criminal purpose of a group and to contribute to the joint criminal enterprise or in any event to the commission of a crime by the group. In addition, responsibility for a crime other than the one agreed upon in the common plan arises only if, under the Case No.: -94-1-A 15 July 1999