WASHINGTON ― Special counsel Robert Mueller told Attorney General William Barr that his letter to Congress at the conclusion of Mueller’s investigation into Russian interference in the 2016 U.S. election mischaracterized the special counsel report.

Mueller complained to Barr that his four-page letter “did not fully capture the context, nature, and substance,” of the special counsel report. The Washington Post first reported on the existence of the letter on Tuesday.

“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 wrote in a March 27 letter.

The revelation came hours before Barr is set to testify on his handling of the probe before the Senate Judiciary Committee. A spokesman for Mueller declined to comment.

Justice Department spokeswoman Kerri Kupec told HuffPost on Tuesday that, in a subsequent conversation between Mueller and Barr, the special counsel “emphasized that nothing in the attorney general’s March 24 letter was inaccurate or misleading” but that Mueller was frustrated by “the lack of context and the resulting media coverage regarding the special counsel’s obstruction analysis.”

House Judiciary Committee Chairman Rep. Jerry Nadler (D-N.Y.) said Tuesday that Barr “should not have taken it upon himself to describe the Special Counsel’s findings in a light more favorable” to President Donald Trump.

Nadler demanded that the Justice Department provide his committee with Mueller’s letter by 10 a.m. on Wednesday, and noted that Barr had “expressed some reluctance to appear” before the House Judiciary Committee.

“It was only a matter of time before the facts caught up to him,” Nadler said of Barr.