WASHINGTON — Robert Mueller objected to Attorney General William Barr's characterization of the principal findings of the Russia investigation and asked the attorney general on multiple occasions to release the special counsel's prepared summaries of the report.

Mueller communicated his objections to Barr in a letter on March 27, three days after the attorney general disclosed the special counsel's conclusions in a summary letter clearing President Donald Trump of having obstructed the investigation into Russian interference in the 2016 presidential election. Trump relied on that summary letter to claim total exoneration.

But in his letter, the first clear window into Mueller's thoughts since he became special counsel two years ago, he said Barr's summary of the investigation's principal conclusions "did not fully capture the context, nature, and substance" of the probe.

"There is now a 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."

According to Mueller's letter and a subsequent statement released late Tuesday, Mueller expressed his differences with Barr at least three times before the full report was released in April. The first time was on March 25, the day after the attorney general released his summary of Mueller’s conclusions. He reiterated those concerns in the March 27 letter, and the two spoke by telephone March 28.

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Mueller first wrote to Barr on March 25, enclosing with the letter an introduction and executive summaries for each of the report's two volumes marked with necessary redactions. Mueller wrote to Barr again on March 27, requesting that the attorney general release the documents to Congress and to the public. "Release at this time would alleviate the misunderstandings that have arisen and would answer congressional and public questions about the nature and outcome of our investigation," he wrote.

For weeks, Barr's summation was the only information available to the public about the outcome of an investigation that shadowed the first two years of Trump's presidency.

Barr's summary letter to Congress told lawmakers that Mueller's investigation had not found that Trump conspired with Russian efforts to sway the election in his favor or that he illegally sought to obstruct the inquiry that followed.

"The special counsel emphasized that nothing in the attorney general's March 24 letter was inaccurate or misleading, but he expressed frustration over the lack of context and the resulting media coverage regarding the special counsel's obstruction analysis," Justice spokeswoman Kerri Kupec said in a statement.

Mueller's full report, released on April 18, offered a more searing account of Trump's conduct than the one offered by Barr. It recounted 10 episodes in which Trump had sought to thwart the inquiry into his campaign's ties to Russia, including details of his demands that his aides fire Mueller or focus his inquiry only on future elections.

Mueller's office said it would have been unfair to attempt to determine whether those actions constituted federal crimes, because the Justice Department has long contended that a president cannot be indicted. But investigators pointedly refused to clear Trump of wrongdoing, saying, "If we had confidence after a thorough investigation of the facts that the president clearly did not commit obstruction of justice, we would so state.”

Kupec said that after Barr called Mueller to discuss his letter. "They then discussed whether additional context from the report would be helpful and could be quickly released," she said.

Barr ultimately determined, Kupec said, that it would "not be productive to release the report in piecemeal fashion."

Read the report:Read special counsel Robert Mueller's report into President Trump, Russian interference

Read the letter:Mueller report: Read Attorney General Barr's summary of the Russia investigation

More:Trump took steps to fire Mueller, stop probe after campaign welcomed Russian dirt on Clinton, Mueller report says

"The attorney general and the special counsel agreed to get the full report out with necessary redactions as expeditiously as possible. The next day, the attorney general sent a letter to Congress reiterating that his March 24 letter was not intended to be a summary of the report, but instead only stated the special counsel's principal conclusions, and volunteered to testify before both Senate and House Judiciary Committees."

Confirmation of that dispute, first reported by The Washington Post, came on the eve of Barr's appearance before the Senate Judiciary Committee to discuss his handling of the Mueller report. Democrats questioned whether the attorney general sought to provide cover for the president.

In the House, Barr has threatened to skip a hearing planned for Thursday over a clash with Judiciary Committee Chairman Jerry Nadler, D-N.Y., over how the attorney general would be questioned.

Nadler has proposed that committee lawyers be allowed to interrogate Barr, in addition to questioning by lawmakers. The attorney general has objected to that format, including a demand that he submit to questioning in a closed session concerning material that was redacted from the public version of the Mueller report.

After Mueller's office declined to reach a conclusion on whether Trump had obstructed justice, Barr and his deputy, Rod Rosenstein, reviewed the evidence investigators gathered and drew their own, saying they saw no crime. That decision has been the focus of mounting questions since the attorney general delivered his four-page summary of Mueller's conclusions last month to the public and Congress.

Barr said he intervened after Mueller did not resolve whether Trump's conduct, which included attempts at curtailing the investigation and a proposal to dismiss Mueller, amounted to a crime.

Mueller's subsequent report examined 10 instances of possible obstruction, including Trump's directive to White House counsel Don McGahn that the special counsel be fired.

Trump has since denied ever issuing such an order. The president declined submit to any questions from the special counsel about his alleged attempts to obstruct the 22-month investigation.

Questions about Trump's alleged obstruction efforts are expected to loom large during Wednesday's Senate hearing.

"This is exactly why I said that Mr. Barr should never have been confirmed in the first place," said Sen. Mark Warner, D-Va., the vice chairman of the Intelligence Committee. "At this point, he has lost all credibility."

Warner urged that Mueller be called to testify as soon as possible.

In the House, Nadler demanded that Barr produce a copy of Mueller's letter before his scheduled appearance there.

"The special counsel’s concerns reflect our own," Nadler said Tuesday. "The attorney general should not have taken it upon himself to describe the special counsel’s findings in a light more favorable to the president. It was only a matter of time before the facts caught up to him."

Late Tuesday, the Justice Department released a copy of Barr's prepared remarks for his Tuesday appearance before the Senate Judiciary Committee in which the attorney general said he fulfilled his confirmation hearing pledge that he would "allow the special counsel to finish his investigation without interference; and second, that I would release his report to Congress and to the American public."

"I believe that the record speaks for itself," Barr's statement says. "The special counsel completed his investigation as he saw fit. As I informed Congress on March 22, 2019, at no point did I, or anyone at the Department of Justice, overrule the special counsel on any proposed action. In addition, immediately upon receiving his confidential report to me, we began working with the special Counsel to prepare it for public release and, on April 18, 2019, I released a public version subject only to limited redactions that were necessary to comply with the law and to protect important governmental interests."

Barr's statement went on to defend his March 24 letter disclosing Mueller's "bottom line conclusions."

"My main focus was the prompt release of a public version of the report so that Congress and the American people could read it for themselves and draw their own conclusions," Barr's statement says. "The department’s principal responsibility in conducting this investigation was to determine whether the conduct reviewed constituted a crime that the department could prove beyond a reasonable doubt. As attorney general, I serve as the chief law-enforcement officer of the United States, and it is my responsibility to ensure that the department carries out its law-enforcement functions appropriately. The special counsel’s investigation was no exception."

On clearing the president of obstruction, Barr maintained that he and Rosenstein "concluded that, under the principles of federal prosecution, the evidence developed by the special counsel would not be sufficient to charge the president with an obstruction-of-justice offense.

"The deputy attorney general and I knew that we had to make this assessment because, as I previously explained, the prosecutorial judgment whether a crime has been established is an integral part of the department’s criminal process," Barr's statement says.