The Federal Bureau of Investigation (FBI) processes thousands of requests each year. When requesting a file on a certain individual under the Freedom of Information Act (FOIA) / Privacy Act, you, as the requester, are required to show either 1) a certified statement from the person you are requesting files on, and the fact they are ok with you receiving the records, or 2) proof of death. This can be in the form of a death certificate or obituary from a valid/reputable source.
As these requests for files are processed, the FBI has compiled a list, used as an internal reference, which totals more than 17,000 people that the FBI understands are deceased.
What is valuable about this list, is that each and every name on it — can be be referenced by researchers, as that individual actually having a file, and that the FBI already has determined and understands, they are deceased.
In November of 2016, The Black Vault received the most recent version of this list (or believed to be the most recent version.) It is archived here for reference.