Here is the government's description of the "smoking gun"' audio recording. It's bad. It clearly shows mens rea for several of the charges and likely fulfills the actus reus for the espionage charges as well (insofar as the recording includes Trump disclosing information within the classified document(s)).

This is so unbelievably reckless. Trump really has no regard for US national security.

One clear theme of the indictment is that Trump was directly involved in the storage of document boxes, including making some of the most indefensible decisions about where and when those documents were stored at Mar-a-Lago.

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OMFG!!! This is a charge unto itself. Trump confessed to the crime as he was completing it.