Washington-area police said they are preparing to combat the “flash mob” phenomenon, a new tactic being used by criminals and violent youths around the country and in the region. Flash mobs are groups that form quickly, often coordinated using social media such as Twitter and Facebook. The earliest flash mobs engaged in more innocent pursuits such as dancing or throwing snowballs, but some groups are now gathering to attack innocent bystanders, fight among themselves or commit other crimes.

About 30 young people suddenly showed up at a 7-Eleven in Germantown at 2 a.m. Saturday and began grabbing merchandise off the shelves. People swiped drinks, candy and ice cream, and then left almost as quickly as they appeared — without paying.