Oh my god, I want to correct my answer in this topic before! I totally disagree with it. I watched GF 2 again yesterday and everything links to Fredo. It was definetly Fredo who killed them.
The best link is the fact that Fredo's mistress runs out the house and says: " Two man.....before my window.....dead!!!!"
And Michael gives an other good link. He says it should be someone nearby who's very afraid when the two people survive and talk.
