The special counsel Robert Mueller wrote to Attorney General William Barr in late March objecting to the way Barr handled the results of Mueller's investigation, multiple news outlets reported Tuesday.

This kind of letter is extremely unusual, and its contents have already proved embarrassing for Barr.

Other members of the special counsel's team reportedly believe Barr spun the report to cast the most favorable light possible on President Donald Trump.

Disclosure of the letter has exposed Barr to further scrutiny and calls for his resignation ahead of scheduled high-stakes congressional appearances.

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Attorney General William Barr and the special counsel Robert Mueller have been friends for decades — so close that their wives reportedly attend the same Bible study and their families socialize together.

But the fallout from Mueller's report on Russian interference in the 2016 US presidential election might place the relationship under strain — as shown by a newly public letter Mueller sent to Barr.

The document, obtained by The Washington Post and quoted in a Tuesday report, exposes the depth of Mueller's concern about how Barr handled the release of the report and concern at the redaction of some of its findings.

The timing of the letter's disclosure — the day before Barr was scheduled to appear in back-to-back congressional hearings — leaves the attorney general exposed to a damaging public interrogation.

Some Democrats are using the letter to demand Barr's resignation even before he appears.

It is unclear whether Mueller had a role in the release of the letter to The Post, and Mueller's representatives have yet to comment. Nonetheless, the release of the letter is having political consequences already.

Per The Post, Mueller in his letter complained that Barr's four-page summary "did not fully capture the context, nature, and substance" of Mueller's 448-page Russia report.

Mueller sent his letter days after Barr released his summary in late March, though it took until this week for it to become public.

Barr with reporters in 1991, when he was President George H.W. Bush's attorney general. Robert Mueller, standing just behind him, was Barr's assistant attorney general. AP Photo/Barry Thumma

President Donald Trump hailed Barr's summary, released weeks before the report itself, as a complete exoneration from claims Trump conspired with Russia to secure victory in 2016 or tried to obstruct the subsequent investigation.

In his letter, Mueller reportedly expressed frustration with his friend for creating what he called "public confusion about critical aspects of the results of our investigation."

Though Mueller's concerns are expressed in relatively soft language, the fact that anything at all was put to Barr in writing is a striking departure from Justice Department norms.

Chuck Rosenberg, who worked closely with Mueller while Mueller was FBI director in the early 2000s, told Politico that officials were "conditioned" not to send written objections to one another and that he had never known Mueller to do so before.

In the summary, Barr said Mueller declined to reach a conclusion over whether Trump obstructed justice. He then detailed his own conclusion that the evidence in the report was not sufficient to accuse the president of obstruction.

But in his letter to Barr, Mueller reportedly requested that the attorney general release the introductions and summaries of the report.

When those were eventually released, it became clear that they painted a damaging picture of Trump's attempts to derail the investigation.

They cite the former White House counsel Don McGahn as saying he had refused orders by Trump to fire Mueller.

Barr is scheduled to testify before the Republican-led Senate Judiciary Committee on Wednesday and before the Democrat-led House Judiciary Committee on Thursday.

He is likely to face interrogation from Democratic lawmakers over revelations from the letter.

"Did Bob Mueller support your conclusion?" Democratic Sen. Chris van Hollen of Maryland asked Barr of his summary when Barr appeared before the Senate Appropriations Committee on April 10.

"I don't know whether Bob Mueller supported my conclusion," Barr replied.

This response now appears at best misleading given that Barr had received Mueller's letter on March 27.

Democrats are even calling for Barr to resign with the contents of Mueller's letter now public.

Van Hollen on Tuesday accused Barr of misleading him in his April 10 testimony.

"I asked Barr, 'Did Bob Mueller support your conclusion?' His answer was, 'I don't know whether Mueller supported my conclusion," the senator tweeted.

—Chris Van Hollen (@ChrisVanHollen) May 1, 2019

"We now know Mueller stated his concerns on March 27th, and that Barr totally misled me, the Congress, and the public," Van Hollen added. "He must resign."

The letter certainly places Barr under increased pressure and will strengthen the position of Democrats who want Trump impeached for the behavior exposed by Mueller.

Reports on the letter came only hours after Joe Biden, who is leading early polls for the Democratic presidential nomination, said Democrats might be forced to impeach the president if he continued to block requests from congressional panels.