two individuals lingering at a bus stop and her cop sense tells her something is up. she recognizes one of the men from an ongoing investigation. >> over by the newspaper stand and the bus stop, the guy leaning down, he's got some merchandise there in a white bag. he's looking at it. the guy in the backpack. there's a deal going on right there. see the guy taking off his backpack now? basically they are transferring the merchandise to these backpacks. whatever he doesn't get rid of it will go somewhere else. once his backpack is completely full, he will leave or have somebody pick up that backpack and give him a new backpack. >> these low level fences with backpacks are basically mules, but investigators must follow them to see who they are selling to. it can be a difficult game to play. >> it's very hard to follow sometimes. it's very hard to follow the merchandise because they are using different modes of transportation, bus, cable car, taxi, bicycle, or on foot.