Is the article correct in stating that pretty much all of the evidence centers around the six letters? IMHO, and apparently the FBI's too, that's a pretty weak connection. It wouldn't be difficult for anyone, especially a CIA or FBI agent, skilled in writing forensics to point the finger at someone else.

So...what evidence was in the book? Because the article has literally zero other than "this guy's ex-boss said he totally recognised a letter".

Does the book actually have any details about the code? I haven't found anything online that looks even slightly convincing.