David Kennedy is a criminologist and professor of criminal law at City University of New York.
He researches crime prevention regarding criminal gangs and his methods have been successfully used in, among other places, 60 cities in the United States.
According to Kennedy, this is always a very small part of a city's population, around half a percent, which accounts for 50-80 percent of the violence.
It is the group dynamics within these circles that one should focus on changing according to his research.
See the interview from Agenda in the clip.