Decided to do some Googling in an attempt to find out what became of these guys and I came across this article. Apparently there was some doubt about whether or not they were guilty of sending the letters, or at the very least whether they acted alone! Check it out...

http://articles.latimes.com/1990-01-30/local/me-829_1_mailing-threatening-letters

Doubt that they were innocent, or that they had accomplices. Seems pretty cut and dried to me. Here's (http://articles.latimes.com/1989-12-07/local/me-335_1_big-ears) another article which says the men would often drive around with surveillance equipment and spy on people.