A dragnet is any system of coordinated measures for apprehending criminals or suspects; including road barricades and traffic stops, widespread DNA tests, and general increased police alertness. The term derives from a fishing technique of dragging a fishing net across the sea bottom, or through a promising area of open water.
While a dragnet can refer to any kind of focused police presence, the traditional definition involves defining an area (e.g., a building, or a city block) and/or category (e.g., those of a specific ethnic group within an area) and conducting at least a brief investigation of each person within. Thus, if a criminal was traced to a specific location, everyone in that location might be searched for incriminating evidence. Since the 1950s, such "dragnets" have generally been held to be unconstitutional, as unreasonable search and seizure actions.
While traditional "stop and frisk" dragnets have largely fallen into disuse, New York's strategy for controlling serious crime by stopping many of those loitering in areas where such loitering is thought to be associated with lesser crimes has been called a dragnet. Similarly, controversy remains over other activities held to be dragnets as well. An active area of legal controversy, for example, is that of warrant-less wiretaps. If all cell phones are monitored by machine for certain words or phrases thought to be associated with terrorism, and the results used to cue focused investigations, the ACLU argues that this constitutes a kind of dragnet.