Seeking records sealed by the FBI

I am not certain what you mean by sealed records, but this is the FBI site that tells one how to make a FOIA request.

https://www.fbi.gov/services/information-management/foipa/requesting-fbi-records

  The only way that I have been able to determine if other agencies had records, was to look at the records from the first agency I found and look at where they got their information or who or what agencies were in the cc list on letters.  Then I would request or look for those agency's records.