The correct answer is criminal activity.
Criminal activity was common in American cities in the mid-1800s.
In that time, the United States received many immigrants. It is suspected that immigration was one of the reasons that increased crime in America. Police departments decentralized its offices in order to have more contact with citizens. This also allowed for better spotting of memebers of crime organizations, to identify potential problems and to have better and faster responses to illicit or violent crime. People started to cooperate more with law enforcement.