That's how investigations normally work.

You take one piece of evidence, combine it with more evidence, and eventually you have a full case.

The Feds have the back book (and so does the internet, because it was leaked online). The feds also have a lot of video that I personally don't want to see, but it should be enough to connect at least a few names.

The fact that they refuse to connect any names means that Trump is in too many of those videos.