Flash mobs are a group of people who assemble suddenly in a public place, usually organized by social media such as emails, Twitter or Facebook. They originally gathered to perform an unusual or entertaining act such as dancing, singing, or throwing snowballs, for a brief time and then disperse. 
Increasingly, though, flash mobs have turned violent. Some groups are now gathering to attack innocent bystanders, fight among themselves or commit other crimes.
Across the country in cities such as Philadelphia, Los Angeles, Cleveland, and Milwaukee flash mobs …