In a Wednesday letter obtained by POLITICO, Bryan Pagliano's lawyer Mark MacDougall told Senate Judiciary Chairman Chuck Grassley (R-Iowa) and Senate Homeland Security Chairman Ron Johnson (R-Wis.) that he would give no such preliminary overview, known in legal terms as a proffer. Both chairmen hoped to get a better sense of what Pagliano knew about Clinton's homebrew server — which he set up in 2009 before she headed to the State Department.

But MacDougall, an attorney at Akin Gump Strauss Hauer & Feld LLP, said such an exploratory discussion of what Pagliano knew had no basis in law and could open up his client to accusations that he "waived his right" to avoid self incrimination.

"Members of congressional committees and their lawyers have lately taken an expansive view of what constitutes a waiver by an individual citizen of his or her right under the Fifth Amendment," he wrote. "Any 'proffer session' or other disclosure by Mr. Pagliano — or his lawyers acting on his behalf — of the contents of his possible testimony creates the very practical risk that our client will later be said to have waived his constitutional protections."