In this movie it was Sammy who killed Nicky

I noticed in this movie it was Sammy who killed his brother in-law Nicky. Is there any truth to this? I know Sammy has said that it wasn't him. But of course he's going to say that. I've spoken to people very familiar with things like this and they have told me that Sammy is full of it when he says he only pulled the trigger once. So not everything Sammy has said is truthful in my opinion. Has anyone ever heard that Sammy was the one who pulled the trigger when Nicky got whacked?