How does Swiss law define undercover investigation (verdeckte Ermittlung, Art. 285a of the Criminal Procedure Code)?

Police officers, or people temporarily employed for police tasks, use a false identity backed by documents (a "legend") and deceptive behaviour to make contact with people, build trust and get into a criminal milieu, in order to solve particularly serious crimes.

The elements of the definition:

• Who: members of the police, or people temporarily hired for police tasks.
• How: a false identity backed by documents (Legende, a cover story with forged papers) and deceptive behaviour.
• What: making contact with people with the aim of building a relationship of trust and getting into a criminal environment.
• Why: only to solve particularly serious crimes.

The last element is the key limit. Deception by the state is so far from normal police work that it is allowed only for the most serious crimes.

Go deeper:

• Wikipedia: Mr. Big (police procedure) — a well-known undercover technique to obtain confessions, and its criticism.