Closed circle of suspects

The closed circle of suspects is a common element of detective fiction, wherein a given crime (usually a murder) occurs within a limited and predefined group of characters, all of whom are isolated in a confined setting such as a remote estate, island, or train, ensuring the perpetrator must be one among them. This narrows the investigative focus to a limited number of suspects, each with credible means, motive, and opportunity.

Source: Wikipedia — Closed circle of suspects (CC BY-SA 4.0)