Go back and rewatch the early episodes of the Hokage battle.

The faces of Zaku and Kin can be seen when the Third finally seals away the souls of the First and Second and the bodies collapse into dust.

The red tags of the back of the kunai embedded into the heads of the First and Second (this is explained by the Anbu watching the battle) make them into puppets, fighting machines controlled by the will of the person who put in the tags.