If I were LEO I imagine I would be torn by the temptation to wade through the wide variety of possibilities and collected data by using nationality-based profiling.
Back to the question of multiple getaway cars, that is certainly a possibility, but the larger the group and the larger the materiel, the more attention one attracts. In the OKC bombing, for example, IIRC many people recalled *two* Ryder trucks and multiple drivers moving and locating together, in some detail. The witnesses probably remembered more rather than less due in part to the size of the operation.
Also the larger the network of operatives working together, the greater the danger of exposure due to the weakest link. Security and secrecy is maximized by keeping the cells as small as possible and operating independently without knowledge of or links with each other.