It is probably illegal for FBI to ask for NSA's help in gaining evidence for a US criminal case, although pointing that out is sure to start a raucous subthread about "parallel construction".

If it's "probably illegal", surely you can cite the law?

Notably, the FISA cell phone record orders passed information to the NSA through the FBI, so no hint of a "chinese wall" there. Secret bulk collection for 'national security', everyone bathing in the same pool of data.

I do suspect it's policy not to casually request, or become overly dependent on, such NSA-to-domestic sharing. And – as we've seen in the "parallel construction" revelations you allude to – it's definitely policy to try to obscure any such sharing when it happens.

But to reassure people that it's "probably illegal" is flimsy hand-waving when you can't name the law, and there's public evidence that it happens to the contrary.

One of the only things that should make you feel optimistic is that there are a bunch of intelligence/law-enforcement agencies and they all hate each other and hate the idea of working together or sharing information with each other.