The letter did not directly state that Mueller disagreed with Barr’s bottom-line conclusions, and a Justice Department spokeswoman told reporters that “the special counsel emphasized that nothing in the attorney general’s March 24 letter was inaccurate or misleading.” Democrats demanded Barr’s resignation anyway.

“We now know Mueller stated his concerns on March 27th, and that Barr totally misled me, the Congress, and the public. He must resign,” tweeted Senator Chris Van Hollen of Maryland, who had asked Barr whether Mueller supported his conclusion that there was insufficient evidence that Trump had illegally obstructed the special counsel’s investigation into whether his 2016 campaign had conspired with the Russians to tip the presidential election.

Read: The complicated friendship of Robert Mueller and William Barr

Mueller’s apparent protest of Barr’s handling of his report is all the more dramatic because, by the attorney general’s own description, the two have been personal friends for more than 30 years after they worked closely together during Barr’s first stint at the Justice Department. Former colleagues described to me how Barr would rib the straitlaced Mueller during their daily morning meetings.

“They both hold the institution in very high regard,” said Paul McNulty, who sat in those meetings when he served as the department’s chief spokesman in the early 1990s. “It’s personal for them both. They have a certain deep affection for the work of the Department of Justice.”

McNulty, who went on to serve as deputy attorney general in the George W. Bush administration, told me last week that while there were disagreements between Mueller and Barr, “it wasn’t personal. They both knew that the other person was the real deal.”

In his letter, which the House Judiciary Committee released publicly on Wednesday morning, Mueller referenced two separate meetings in which his team advocated for the release of the report’s own executive summaries. The clear implication is that Mueller wanted those documents to serve as the official public summaries of his findings. Barr, however, chose to write his own letter and to withhold the summaries until he released the full, redacted report weeks later.

On Wednesday, Barr reminded senators that it was his prerogative.

“His work concluded when he sent his report to the attorney general,” he said of Mueller. “At that point, it was my baby, and I was making a decision as to whether or not to make it public.”

Mueller’s decision to write Barr a confidential letter is consistent with the way he handled internal disputes at the FBI, according to Frank Figliuzzi, who served as the bureau’s counterintelligence chief for a time under Mueller. Figliuzzi told me last week that when Mueller was overruled by leaders at the Department of Justice on a case, he would write a confidential memo memorializing his views and send it up the chain. “He wasn’t the one who would yell, scream, bang on the desk, and say, ‘This is all wrong,’” Figliuzzi said. “Those rare examples were very illustrative of him playing within the parameters he was given, but yet asserting his principles and ethics when necessary.”