Ku Klux Klan bit

Yes, the movie doesn't specifically mention the murders' religion, but both were Jewish and much hatred attended the trial. While one might expect that type of reaction to a case concerning the murder of a child, the anti-Semitism exhibited at the time of the trial was palpable. One has to wonder how much worse things would have been if the victim had not also been Jewish.

I believe it also alluded to the fact that Orson Welles character was a well known atheist and everyone knows that the KKK use crosses because they believe that they are good God-Fearing Christians who are doing God's work when they lynch people and burn crosses.

It's also a centuries-old belief that Jews sacrificed Christian children, which was often an excuse for pogroms and murders of Jews. This might have rung a bell with people who hated Jews anyway.