U.S. Attorney General William Barr can be expected to face questions Wednesday from members of the U.S. Senate Judiciary Committee about the nature and scope of his communication with Robert Mueller III, the special counsel who was appointed to lead the Russia investigation.

Mueller was frustrated about how Barr in late March portrayed the “principal conclusions” of the report, which was released in redacted form last month. In a letter to Barr on March 27, the substance of which was released late Tuesday, Mueller said the attorney general's four-page summary “did not fully capture the context, nature, and substance of this office's work and conclusions.”

“There is now public confusion about critical aspects of the results of our investigation,” Mueller wrote. “This threatens to undermine a central purpose for which the Department appointed the Special Counsel: to assure full public confidence in the outcome of the investigations.”

Mueller's letter was excerpted widely Tuesday night in media reports, but the full letter was made public Wednesday morning—as Barr prepared to testify about Mueller's report before the Senate Judiciary Committee. The appearance is Barr's first since the release of a redacted version of Mueller's 448-page report.

Barr drew criticism in March for releasing a four-page summary that largely characterized Trump as being cleared by Mueller's special counsel team. Mueller did not find the Trump campaign conspired with Russian actors, and the special counsel did not make a recommendation on whether Trump tried to obstruct the investigation. Still, Mueller's report was more damning than Barr presented it in a letter to Congress.

Mueller in his letter urged Barr to release certain materials that Mueller said would “alleviate the misunderstandings that have arisen and would answer congressional and public questions about the nature and outcome of our investigation.”

After receiving Mueller's letter, Barr called the special counsel to talk about it, the Justice Department said. Kerri Kupec, a DOJ spokesperson, described the call as “cordial and professional.”

Mueller “expressed frustration over the lack of context and the resulting media coverage regarding the Special Counsel's obstruction analysis,” Kupec said.

“They then discussed whether additional context from the report would be helpful and could be quickly released. However, the Attorney General ultimately determined that it would not be productive to release the report in piecemeal fashion,” Kupec said.

Mueller's letter to Barr is posted in full below:

Ellis Kim contributed reporting from Washington.