The phenomenon of the flash mob — masses of people organized using social media — began innocently enough and quickly mushroomed.
Among the highlights in recent years: One hundred people danced to Michael Jackson's "Thriller" in Times Square; 1,500 people took part in a pillow fight in San Francisco; 2,000 hardy souls slung snowballs in Washington, D.C.
Yet as these experiments in public spontaneity gain popularity, some of them are becoming troublesome.
In a number of cities, gatherings have grown unruly and led to vandalism or assaults — and worse. Police have described some as "mob thefts" in which people assemble outside retail and convenience stores, loot them and vanish before police arrive.
Police and social media gurus say these flash mobs usually are organized on Twitter, Facebook and through text messages, as are peaceful flash mobs. Police say some of the thefts, in particular, appear to have been coordinated in advance but that violence and vandalism at other gatherings may have been random.
Still, the real-time immediacy and apparent randomness of the incidents can combine to outwit conventional policing methods, according to law enforcement authorities.
"Traditionally, if folks are assembling or holding a protest, there are permits or processes in place for law enforcement to prepare and be on site in case things happen. But with flash mobs, there's no advance warning. Law enforcement might not have staff on hand," says Nancy Kolb of the International Association of Chiefs of Police.
Most incidents have occurred in large urban areas with busy retail or tourist corridors. Police say the participants almost always are teens and young adults — those most likely to move in large groups and tweet or text message often as a primary method of communicating.
Prelude To A Mob, Unfolding 140 Characters At A Time
"There's probably no one way these mobs are being organized, but without too much doubt I would say that Twitter is going to be involved, because Twitter is in the moment," says Lauri Stevens, a social media consultant to law enforcement agencies.
Stevens is the founder of the annual Social Media, the Internet and Law Enforcement conference, which helps police departments from across the country make better use of social media to investigate crimes and communicate with the public. This year's gathering was held earlier this month in Chicago, where mobs recently looted stores on the city's famed Magnificent Mile. Local media reports said the flash mobs there were coordinated through text messages.
The most violent of recent incidents occurred in April when a man was shot twice at a flash mob in Venice Beach, Calif., one of the largest and busiest tourist destinations in the state. Police haven't said whether the shooting was plotted in advance, but plans for the gathering unfolded hour by hour on Twitter. The tweets caught the attention of a watchful resident.
As a founding member of a Venice Beach neighborhood watch group, Alexandria Thompson monitors Web discussions about her neighborhood, using TweetDeck, an application that organizes social media platforms such as Twitter, Facebook and LinkedIn as a single, customizable view for users. TweetDeck flags all mentions of "Venice Beach" on Twitter and sends them to Thompson.
On the night of April 15, TweetDeck sent Thompson a tweet saying "Venice beach bball ct going up tomorrow." A stream of retweets followed, without offering a specific reason for the gathering. Thompson decided to click on one of the users' profiles and ask to follow the person. She said she was immediately approved and found a flurry of traffic between users who appeared to be teenagers by their profile pictures.
The tweets set a meeting time of 4 p.m. April 16, according to a review of the message traffic. The exchanges also included a running dialogue between two users touting rival gang affiliations.
"Out here, you hear gang anything and it's nothing to joke about," Thompson said. "So that really concerned me."
When the Twitter traffic continued past midnight and into the late morning, Thompson sent a text message to a Los Angeles police officer on the Venice Beach beat, warning him of a possible flash mob coming to the boardwalk.
That afternoon, crowds began to appear next to the boardwalk's basketball courts.
"Within half an hour, 45 minutes, you had 300 to 500 people arriving," said police Sgt. Marc Reina, who supervises the area. "They weren't all gang members. Based on their clothing you had people who could have been associated with gangs — the reds and blues, for Bloods and Crips."
Policed called for more officers, but the reinforcements couldn't prevent tensions from boiling over. About 6:30 p.m., several gunshots erupted. A man was struck in the head and the chest area and later was listed in critical condition at a nearby hospital, Reina said.
In the aftermath, Reina said, the police department has begun to more closely monitor Twitter and other platforms.
Reina said police are using the tweets as part of the shooting investigation.
"It's new for us," Reina said. "We've monitored websites for large events coming up, but this is a first for me. It's something where the texts or tweets go out right away; you have to react fast."
Flash-Mobbing Problems Across The Country
Philadelphia was the site of several incidents that drew early attention, between 2009 and 2010. It began with teenagers gathering downtown, then sprinting down streets, fighting with each other, assaulting pedestrians and vandalizing storefronts. Police say high school students have used Facebook and other media to gather in Center City and elsewhere.
In February, at a convenience store in St. Paul, Minn., about 50 young people overpowered the employee at the door and looted the store. The incident was recorded by a store camera. Last fall, about 20 youths overran another convenience store and assaulted the clerk as they bolted with stolen items, according to St. Paul police, who said social media might have been used in advance.
Last month in Washington, D.C., nearly 20 youths gathered outside the G-Star Raw clothing store in Dupont Circle and filed in together, brushing past customers. Video from the store's security camera shows them marching directly to the shelves of expensive designer jeans and racks of high-end shirts.
They sorted through the selections for their sizes and tucked them under their arms, initially behaving like usual, if rushed, customers. Then they all suddenly made for the exit, escaping before police arrived 10 minutes later. In just moments, on a busy street in the middle of the day, the suspects had stolen an estimated $20,000 in merchandise, police said.