A sleeper cell is a type of clandestine cell system used by terrorist organizations, resistance fighters, and other groups to make it harder for authorities to catch them. A sleeper cell is a group of operatives, spies, or terrorists who live in secret among a targeted community, waiting for instructions or an opportunity to act. Sleeper cells are designed to remain dormant until they receive orders or decide to act. They are often led by a charismatic true believer who believes that any act of violence is moral if it serves their cause. Sleeper cells are difficult to detect because their members are often unknown to each other and to the authorities. They can be found in many different types of organizations, including terrorist groups, criminal organizations, and foreign intelligence services. Sleeper cells are a significant threat to national security, and experts warn that they may already be present in critical infrastructure such as the energy grid.