This is easy. Carlo Rizzi died the same day as all the other Corleone enemies died, but law enforcement either did not or could not bring anyone "to justice." Everyone suspected that Michael killed Carlo, even Connie, remember she came running into the house screaming "Read the papers read the papers!!!" This is what prompted Kay to ask Michael whether or not he had killed Carlo and of course he told her no.

As for Fredo, it was a drowning accident pure and simple. They just never found the body at the bottom of Lake Tahoe. In GFIII after Michael tells Connie that he has confessed his sins, Connie makes a comment about "poor Fredo" and his "drowning like that." This is her way of saying that she forgives Michael for killing his brother.