I always thought he said "all". And what's the signifigance? A man who is greiving makes an impulsive statement. Of course he wants to know who, what, and why. I think the other Dons would be suspect if the Corleones didn't snoop around looking for answers.

But regarding the validity of the "script", there are mistakes...such as "Carmine Corleone" instead of "Carmine Cuneo". So I could see the "no" being substituted for "all".