Much attention has been paid to Barr’s refusal to commit to sharing the entirety of Mueller’s final report with the public, let alone with Congress, on the ground that the report might be subject to applicable rules of secrecy and privacy. In fact, Mueller’s report could well be of limited value to any congressional inquiry into Trump’s conduct. Under the governing regulations, a special counsel’s final report is supposed to be limited to a discussion of his decisions whether or not to charge particular individuals. And given that Mueller lacks authority to prosecute Trump, the report may well contain limited material about the president. But Barr’s recent statements also suggest that he may argue that the DOJ cannot divulge the information that Congress most urgently requires: the actual testimony and other evidence prosecutors have gathered regarding Trump’s conduct.

It is true that, in most criminal cases, applicable rules impose high hurdles to sharing evidence from criminal investigations with the public or with Congress, the most important among them being the rules governing grand jury secrecy. Much of the evidence in federal criminal investigations, including witness testimony, is obtained primarily in grand jury proceedings. Federal Rule of Evidence 6(e) codifies traditional rules of grand jury secrecy, and–subject to certain specified exceptions–generally prohibits the government from disclosing grand jury evidence outside of indictments (and other charging documents) and criminal trials. Accordingly, as former U.S. attorney Joyce Vance recently explained, the rule could well be construed to prevent any disclosure of Mueller’s evidence.