He was definitely the killer. Before Chris says anyone had died all he indicated is "Remember Dickie Moltisanti" ? The cop responds with "Listen I don't know who told you I was responsible for the death of your father" indicating that he had in fact killed Dickie because Chris never said he was dead. Than its undeniably confirmed when the cop breaks his own bannister and hides behind the couch screaming "I'm sorry" with clear indication that he did it.