Complex sentence: Organised crime was widely believed to control the production and distribution of the films, which was concentrated in New York and involved a core group of around thirty performers that including several trained actors, capable of handling dialogue when required.
Split into simpler sentences: Organised crime was widely believed to control the production and distribution of the films, which was concentrated in New York. A core group of around thirty performers appeared in the films, sme were that included several trained actors and capable of handling dialogue when required.