It's the reasoning behind the press conference that tells infinitely more. While shutting down the investigation, Mueller tried desperately to convict Trump in the court of public opinion. He was leaving the door open to madness and imagination that Trump committed crimes in the hope that it would continue for much longer. Contrast this press conference with the one where James Comey exonerated Hillary. He laid out a concrete case for indictment and even arrest, and then towards the end, said he couldn't do anything about it and that she was not going to have any consequences. See the injustice! This press conference was historic. Bad actors corrupted the justice system, and Mueller just proved it.