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In espionage, agents under non-official cover (NOC) are operatives who assume covert roles in organizations without official ties to the government for which they work. Such agents or operatives are typically abbreviated in espionage lingo as a NOC (pronounced knock). These agents are also known as "illegals" (see Clandestine HUMINT operational techniques). Non-official cover is contrasted with official cover, where an agent assumes a position at an otherwise benign department of their government, such as the diplomatic service. This provides the agent with official diplomatic immunity, thus protecting them from the steep punishments normally meted out to captured spies. Upon discovery of an official cover agent's secret hostile role, the host nation often declares the agent persona non grata and orders them to leave the country.

Agents under non-official cover do not have this "safety net", and if captured or charged they are subject to severe criminal punishments, up to and including execution. Agents under non-official cover are also usually trained to deny any connection with their government, thus preserving plausible deniability, but also denying them any hope of diplomatic legal assistance or official acknowledgment of their service. Sometimes, entire front companies or strawman entities are established in order to provide false identities for agents.

An agent sent to spy on a foreign country might, for instance, work as a businessperson, a worker for a non-profit organization (such as a humanitarian group), or an academic. For example, the CIA's Ishmael Jones spent nearly two decades as a NOC.

Many of the agents memorialized without names or dates of service on the CIA Memorial Wall are assumed to have been killed or executed in a foreign country while serving as NOC agents. In nations with established and well-developed spy agencies, the majority of captured non-native NOC agents have, however, historically been repatriated through prisoner exchanges for other captured NOCs as a form of gentlemen's agreement.

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