Police departments in several cities around the country are investigating what appear to be incidents of "flash mob"-generated violence, in which packs of dozens or even hundreds of youths appear seemingly out of nowhere to commit assaults, robberies and other crimes against innocent bystanders. The motive and circumstances surrounding the attacks that have resulted in numerous arrests around the country are being investigated -- and law enforcement officials in at least one city are looking into a possible racial component to the crimes. Philadelphia Mayor Michael Nutter, who recently imposed a stricter curfew in response to the city's latest attack,...