Read it here.
Here's the Billboard article on the long-secret file. It tells us that the 10-page file contains 2 letters — both from people who were concerned — based on what they'd read in book or seen in a movie — that the death was not a suicide but a murder. The file also contains the letter the FBI sent in response to each letter, telling them that murder is usually a matter of state and local law and that without "specific facts... to indicate... a violation of federal law," there's no basis for an FBI investigation.
There's a little bit more to it, but basically that's it.
ADDED: What stationery do you use when you write to the FBI?
