As stores around the nation gear up for Black Friday, there is one thing on the minds of many retailers: flash-mob robbers.

Groups march into stores and within a few minutes steal thousands of dollars of merchandise, the Post reports. Typically made up of around 15 members, groups burst into a shop and head for the most lucrative items they can find and quickly loot the shop for all its worth. Some flash-mob groups are so skilled that by the time they are noticed, they have already escaped with their bounty.

Flash-mob robbers do not solely relegate their plunders to Black Friday. A group of flash-mob teens stole 19 iPhones over $13,000 at an Apple store in Boston in October.  Another group of young adults raided a Georgetown clothing store in D.C. in November, 2015, stealing thousands in merchandise and causing a great deal of public unrest.

Police officers who have responded to multiple flash-mob robberies say that the longest they have ever seen an attack last is two minutes, the Post reports. In that time frame, robbers can steal as much as $40,000 in merchandise. Given that the average response time for police to a crime scene in the U.S. is 11 minutes, the success sense of flash-mob groups is easy to understand.