Special counsel Robert Mueller wrote a letter in late March objecting to Attorney General William Barr's four-page summary of the conclusions of the investigation into possible ties between Russia and the Trump campaign, a Justice Department official confirmed Tuesday night.

The letter written to Barr expressed the special counsel's frustration that the attorney general's memo to Congress "did not fully capture the context, nature, and substance" of the investigation, according to The Washington Post.

Days earlier, Barr had written that Mueller's 448-page report did not establish a conspiracy between Russia and the Trump campaign. He also said the probe reached no conclusion about possible obstruction of justice by President Trump.

The revelation of differences between Mueller and Barr over the handling of the investigation's conclusions comes a day before the attorney general is scheduled to appear before the Senate Judiciary Committee. Barr is also slated to appear before the House Judiciary Committee on Thursday. The attorney general is expected to come under tough questioning from Democrats on both panels about his summary which the administration has used to argue that Mueller had cleared Trump of any wrongdoing.

The Barr summary has lead to "public confusion," Mueller wrote as quoted by the Post.

" 'The summary letter the Department sent to Congress and released to the public late in the afternoon of March 24 did not fully capture the context, nature, and substance of this office's work and conclusions,' Mueller wrote. 'There is now public confusion about critical aspects of the results of our investigation. 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 and Barr spoke in a phone call the day after the letter was received, said Justice Department spokeswoman Kerri Kupec.

"In a cordial and professional conversation, the Special Counsel emphasized that nothing in the Attorney General's March 24 letter was inaccurate or misleading," Kupec said in a statement. "But, he expressed frustration over the lack of context and the resulting media coverage regarding the Special Counsel's obstruction analysis."

Kupec added: "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. The Attorney General and the Special Counsel agreed to get the full report out with necessary redactions as expeditiously as possible."

When the redacted Mueller report came out in mid-April, there was new evidence of obstruction by the president. "This report does not exonerate the president, but it does point out there's some difficult legal issues in play here," NPR's Carrie Johnson reported.

A spokesperson for Mueller, Peter Carr, declined to comment.

Copyright 2019 NPR. To see more, visit https://www.npr.org.