Comment: Since You asked

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Since You asked

The U.S. Supreme Court has recognized that the duties of law enforcement may require limited, officially sanctioned deception in the course of criminal investigations. United States v. Russell, 411 U.S. 423, 434 (1973). (“Criminal activity is such that stealth and strategy are necessary weapons in the arsenal of the police officer.”)

There's plenty more specifics in the police training manuals if you care to look.