What train of thought did FFC want first time viewers to follow with regards to the question of who tried to kill Micheal?

At first, the viewer thinks it was Frank following Micheal's conversation with Roth. Then we are led to believe that it was Roth, after Micheal's meeting with Frank in NY. Then, when Frank is garotted - 'Micheal Corleone say hello', are the audience meant to think that it was actually Frank all along? Or is it purposely left ambiguous? What were you thinking the first time you saw the movie?