The government sometimes takes the position that even acknowledging that someone was involved in or the target of a criminal investigation would be a privacy violation. Courts have in some occasions sanctioned the practice of providing a “Glomar” response in such situations.
If you receive a Glomar response under Exemption 7(C) that refuses to confirm or deny the existence of records, you may attempt to overcome it by asserting an overriding public interest, such as those described above.