What about this:
The FBI suspects there might be others involved. They might even have a hint of and idea who, but from all the evidence they currently possess, there's clear not enough left to support a criminal charge.
It is extremely unethical for the DOJ or FBI to level any sort of accusations if they're not doing do in the form of investigations and charges. The evidence might be gone, or protected by some sort of judicial or executive privilege ruling that can't lawfully be broken by whoever knows what is there.
It may be a giant mess where the criminals had the influence to have the evidence legally buried (if not destroyed.