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.
[1] Since the 1950s, such "dragnets" have generally been held to be unconstitutional as unreasonable search and seizure actions in the United States.
If all cell phones are monitored by machine for certain words or phrases thought to be associated with terrorism, and the results are used to cue focused investigations, the American Civil Liberties Union argues that to constitute a kind of dragnet.
[3] A large, highly publicized dragnet occurred in and around Boston after the 2013 marathon race bombing.
A dragnet that caught worldwide attention was conducted by French law enforcement after the Charlie Hebdo shooting, which killed twelve people in January 2015.