Source for that?

B(5) exemption on FOIA request according to the FBI: Inter-agency or intra-agency memorandums or letters that would not be available by law to a party other than an agency in litigation with the agency.

https://www.fbi.gov/services/information-management/foipa/foia-pa-overviews-exemptions-and-terms

B(5) exemption on FOIA request according to FOIA.gov : Privileged communications within or between agencies, including those protected by the:

1. Deliberative Process Privilege (provided the records were created less than 25 years before the date on which they were requested)
2. Attorney-Work Product Privilege
3. Attorney-Client Privilege