Sidney Powell, a firebrand lawyer whose pugnacious Fox News appearances had earned her numerous private phone conversations with President Trump, sent a letter last year to Attorney General William P. Barr about her soon-to-be new client, Michael T. Flynn.

Asking for “utmost confidentiality,” Ms. Powell told Mr. Barr that the case against Mr. Flynn, the president’s former national security adviser who had pleaded guilty to lying to the F.B.I., smacked of “corruption of our beloved government institutions for what appears to be political purposes.” She asked the attorney general to appoint an outsider to review the case, confident that such scrutiny would justify ending it.

Mr. Barr did what she wanted. He appointed a U.S. attorney six months later to scour the Flynn case file with a skeptical eye for documents that could be turned over as helpful to the defense. Ultimately, Mr. Barr directed the department to drop the charge, one of his numerous steps undercutting the work of the Russia investigation and the special counsel, Robert S. Mueller III.