Flash mobs -- usually announced online in social networking sites, or by e-mails or text messages -- were once benign and entertaining, but recent gatherings by groups of teenagers have evolved into more sinister actions.
Earlier this month, Philadelphia Mayor Michael Nutter signed an order moving curfews to 9 p.m. on Fridays and Saturdays for people younger than 18 in Center City, the heart of Philadelphia's downtown, and in University City, home to the University of Pennsylvania and Drexel University.
Nutter announced the earlier curfew following a string of mob attacks by young people alerted to gatherings via e-mail and social media.
Parents and minors face hefty fines if caught violating the new rules.
Violent "flash mob" attacks have also been reported recently in other cities across the country, leading to crackdowns on curfew enforcement and stepped-up police patrols.
Extra state troopers were ordered in after what was described as a "mob beating" took place at the Wisconsin State Fair.
Attacks in Cleveland, Chicago and Washington, D.C. have all led to the arrests of dozens of teens and resulted in extra police patrols in and around these cities.
Montgomery County Police said Saturday's "flash mob" theft was the first such incident in their jurisdiction, but Starks admits he is concerned about the growing trend.
"I assure you we're taking this crime very seriously," Starks said.
In England, where riots erupted earlier this month, authorities say social networking sites and mobile messaging services were used as tools to organize looting and violence.