The reporter who heard it stands by his story that he heard someone shout it but could not see the person, so there is evidence that it occurred, but no one else has come forward to say they heard it so the investigation is being closed.
And since it was a McCain/Palin rally and they are very choosy about who gets to attend, only loyal supporters are allowed. I'm not sure a loyal supporter would rat someone out to the FBI.
Just my opinion