Retailers and law enforcement around the country are ramping up security to combat a wave of thefts by criminal groups during the busy holiday shopping season.

Some of the most brazen crimes, sometimes known as “flash mob” robberies, have happened in recent weeks. In suburban Minneapolis dozens of people stole goods from three Best Buy stores over Thanksgiving weekend. The week before, approximately 90 people stole goods from a Nordstrom near San Francisco, according to the local police department. Stores from Chicago to Los Angeles have been hit.