During his lifetime, Casey Kasem had millions of fans around the world. Most of them were like you and me: normal, well adjusted, and conscious of the true nature of the relationship between radio DJ and listener--it's friendly, but it's not the same kind of relationship you have with family members, friends, and other people you encounter in the real world. Some of Casey's listeners, however, were not content to stay within the boundaries of the listener/DJ relationship. Some of them crossed the line, and they did so in ominous nways that got the FBI involved. The agency recently released a number of documents describing its investigations into those cases.
In 1980, a woman who said she was president of the Missouri chapter of the Casey Kasem Fan Club paid a visit to the studio where American Top 40 was recorded. But she was not just a fan of the show--she was in love with Casey, and when he married late that year, the woman became very angry. She wrote letters threatening to kill Casey, his new wife, and his children. The threats were reported to the FBI, which didn't have to look too hard to find the woman, considering she put her real return address on the threatening letters. After it was determined she was seeking psychiatric care, the case was closed.
In 1984, another fan wrote to Casey asking for a Long Distance Dedication. After weeks went by and he didn't hear it, he wrote to Casey again, not threatening him directly, but talking about the number of nasty ways in which a person could die. That fan was traced to Mississippi, where agents determined he was not capable of carrying out any sort of action against Casey.
In the early 90s, Casey got into a dispute with a group called Negativland, which used an outtake from American Top 40 in one of their songs. When he asked them to cease and desist, they were unwilling to do so. Over a period of months, the dispute escalated and the FBI was called in. (The ins-and-outs of that case are too complicated to discuss in a short blog post; you can read more about it here.)
If you'd like to read the whole FBI file on the three cases, including copies of the threatening letters (which are pretty creepy), it's here.