Iowa Code § 804.10: Use of force in arrest by private person.

1. A private person who makes or assists another private person in making a lawful arrest is justified in using any force which the person reasonably believes to be necessary to make the arrest or which the person reasonably believes to be necessary to prevent serious injury to any person.

2. A private person who is summoned or directed by a peace officer to assist in making an arrest may use whatever force the peace officer could use under the circumstances, provided that, if the arrest is unlawful, the private person assisting the officer shall be justified as if the arrest were a lawful arrest, unless the person knows that the arrest is unlawful.