As you saw in the film, Vito Corleone's shooting was big news. And that big scene outside the hospital would have been reported, even if the newspapers didn't report that McCluskey hit Michael. So, when McCluskey and Sollozzo were killed by an "unknown" person at their dinner table, everyone would have guessed that the murders were connected to Corleone business. And when Michael, well-known war hero, turned up missing, the finger would point at him. That's why he couldn't come back from Sicily until someone else took his place as the admitted murderer. It happened as Don Cardi described--one of the best passages in the novel that was, alas, not put in the film.