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Thanks for your question. You'll need to file a Freedom of Information Act (FOIA) Request with the FBI. For those who are deceased you'll need to provide some proof of death.
More information and contact information can be found on the FBI's Requesting FBI Records page. https://www.fbi.gov/foia/requesting-fbi-records
Best of luck.
Dear Mr. Rosener,
You can email your search request to firstname.lastname@example.org the individual's full name, date of birth, place of birth, employment agency and time of employment. Though the Department of Justice retained ownership of some FBI records relating to agents, we do hold some civilian employee records relating to early FBI service and all agency employee records. The National Cash Register is a private corporation, though, and not federal employment. As much of that information as possible helps us to determine that we have the correct individual. Thank you!
National Archives at St. Louis