The first question and answer says:
Q.1: How to do Genealogical Research using FBI Files?
A.1: The FBI maintained files on millions of Americans and others from 1908 until the present. Many genealogical researchers have been reluctant to request a copy of a relative's FBI file for research purposes. Here's how to do just that: Just send a simple letter of request to the FBI. Contrary to popular belief, you don't need to have a fancy form or notarized letter. No, the FBI doesn't "start a file" on you when you ask for records that you are entitled to request. Send the letter to:
FBI Freedom of Information Act Unit
Records Resources Division
Federal Bureau of Investigation
9th & Pennsylvania Avenue N.W.
Washington, DC 20535 USA
The letter should include the full name or names of the person whose file you wish to receive, the date of birth, the date of death, the place of birth. Finally, you need to send a photocopy of proof of death. This can be ANY of the following: newspaper obituary, citation from Who Was Who in America, death certificate, biographical reference, encyclopedia or magazine article on the person, or your simple statement that the person was born over 110 years ago. If the latter is the case, then the FBI doesn't require written proof of death because they assume anyone over 110 is probably deceased. But you'll still need the birth date.
You may think: Why would I want the file if I already have the name and birthdate? Well, the FBI file, if there is one, may have all sorts of other data that will help you in your research.
Read the whole thing for cost, waiting time, and other information.
Have you checked your deceased 20th century ancestors who might have had a file? You might be surprised!