This study analyses the behavior in a repeated public goods game when subjects know about the possibility of existence of strict conditional cooperators. We employed a baseline treatment and a threat treatment in which subjects are informed about the possibility of being in a group together with automata playing a grim trigger strategy. We conjecture the resulting game allows for almost fully efficient outcomes. Contributions in the threat treatment increase by 40% before a surprise restart, and by 50% after the surprise restart. In line with the grim trigger strategy subjects contribute either all or nothing in the threat treatment.