Complex sentence: A clandestine weapons store said to be controlled by the Banda della Magliana was kept in the basement of a government building, the NAR had access to it, and ammunition thought likely to have to have come from it was used to kill Carmine Pecorelli.
Split into simpler sentences: A clandestine weapons store of the Banda della Magliana was kept in the basement of a government building, the NAR had access to it. Ammunition thought likely to have to have come from the joint NAR/Banda della Magliana cache was used to kill Carmine Pecorelli.