Mueller had an ethical obligation to report to congress exculpatory evidence persuiant to rule 3-5.4 of the federal prosecutors code of conduct.  As far as I know that never happened.  The question remains, what did Mueller know, and when did he know it?  He certainly appears to be in significant material breech of the rules of his office.

"Standard 3-5.4            Identification and Disclosure of Information and Evidence

    (a)  After charges are filed if not before, the prosecutor should diligently seek to identify all information in the possession of the prosecution or its agents that tends to negate the guilt of the accused, mitigate the offense charged, impeach the government’s witnesses or evidence, or reduce the likely punishment of the accused if convicted. 

    (b)  The prosecutor should diligently advise other governmental agencies involved in the case of their continuing duty to identify, preserve, and disclose to the prosecutor information described in (a) above."