No one really knows who killed the assassins,could have been someone who knew about it(most likely) or someone who saw them on the grounds trying to escape after the alarm had been sounded(unlikely)It was not however,Fredo,who killed them.
Turnbull's answer to the "MC says hello",which I believe,is that it was not meant for Frankie,who they truely intended to kill, but they said it for Richie, the bartender.Since he wasnt a mafioso,he wasnt necessarily under omerta,and could tell the cops that the assassins were sent from Mike,to mislead them.Sorry if I stole your thunder TB