Could the third man have been hiding in Fredo's House?

When Deanna is brought outside she is screaming about somebody being right outside her window. This suggests some complicity by Fredo. I cannot believe Fredo would
be the one who killed them and I dont think Roth would have entructed something like that to him.

But Michael's comment points to a third member of the hit team because when he says he believes the hitters "are prpbably dead already" he then makes a comment that the hit was "botched" and that someone was very afraid by that point.