If you suddenly start noticing police officers around every corner, you might be watching a dragnet in progress. A dragnet is a coordinated attempt to catch a criminal.
Dragnet originally referred to a kind of net for catching fish. It can now also indicate a less literal type of net: one that catches crooks. Police may use a dragnet if they know a wanted criminal is in a certain neighborhood. When cops set up a dragnet, they may call in extra officers, fence off areas from civilians, and arm themselves heavily. This word became famous because of the TV show Dragnet, which featured Sergeant Joe Friday.