i saw a documentary on INS on the travel channel. it mentioned how INS puts several agents to intermingle with the crowd to find people who match descriptions of people they expect to see, or to find people who they might expect to be committing a crime (e.g. drug trafficking or any other smuggling). for example, a man travelling alone, wearing a suit carrying a briefcase, and pushing several large duffel bags looks suspicious.

however, it should be noted that INS agents are allowed to racially profile; it is part of their job description.