Special counsel Robert S. Mueller III wrote a letter in late March complaining to Attorney General William P. Barr that a four-page memo to Congress describing the principal conclusions of the investigation into President Trump “did not fully capture the context, nature, and substance” of Mueller’s work, according to a copy of the letter reviewed Tuesday by The Washington Post.

The letter and a subsequent phone call between the two men reveal the degree to which the two longtime colleagues and friends disagreed as they handled the legally and politically fraught task of investigating the president.

At the time Mueller’s letter was sent to Barr on March 27, Barr had days prior announced that Mueller did not find a conspiracy between the Trump campaign and Russian officials seeking to interfere in the 2016 presidential election. In his memo to Congress, Barr also said Mueller had not reached a conclusion about whether Trump had tried to obstruct justice, but that Barr reviewed the evidence and found it insufficient to support such a charge.

Justice Department officials said Tuesday that they were taken aback by the tone of Mueller’s letter, and that it came as a surprise to them that he had such concerns. Until they received the letter, they believed Mueller was in agreement with them on the process of reviewing the report and redacting certain types of information, a process that took several weeks. Barr has testified to Congress previously that Mueller declined the opportunity to review his four-page memo to lawmakers that distilled the essence of the special counsel’s findings.