If I put an electric light depicting a cartoon character on a sidewalk, the "threat" is generated entirely within the fervid imagination of the terrorized.
Do you at all see the difference??
NO ONE writes "Anthrax" on the cover ~ they let you open the letter and then you learn about it.
Letters, in and of themselves, are hardly threatening. We all get them every day.
Now, when it comes to strange looking boxes with batteries and wires just abandoned on the street, you will notice that we are frequently warned about such things ~ particularly if they have "wires" and "batteries".
Try a google.com search for packages wires batteries threat
"Is that why we don't get any more fan mail?"