Robert Mueller complained to William Barr about how he characterized the Russia investigation

Reporting from Washington —  Special counsel Robert S. Mueller III complained to Atty. Gen. William Barr that Barr’s initial letter to Congress about the Russia investigation did not “fully capture the context, nature and substance of this office’s work and conclusions,” a Justice Department official said Tuesday.

Mueller also wrote that Barr’s four-page letter on March 24, two days after Mueller had filed his final report, sowed “public confusion about critical aspects of the results of our investigation,” said the official, who spoke on condition of anonymity to discuss the internal debate.

The special counsel’s protest, filed in a letter to Barr on March 27, reflects clear tension between the special counsel and the attorney general over Barr’s handling of Mueller’s 448-page report into Russia’s interference in the 2016 election and potential obstruction of justice by President Trump.

RELATED: Read the letter Robert Mueller sent to Attorney General William Barr »

The existence of Mueller’s letter was first reported by the Washington Post.

Barr is expected to be grilled Wednesday by Democrats at a hearing before the Republican-controlled Senate Judiciary Committee that will delve into his handling of Mueller’s report, as well as the attorney general’s conclusion that President Trump did not obstruct justice.

Barr is also sure to be questioned about his comments at an April 18 news conference that put Trump’s actions in a favorable light.

Read Atty. Gen. William Barr’s prepared remarks ahead of Senate Judiciary Committee hearing »

When he received Mueller’s letter on March 27, Barr spoke to Mueller by phone, according to Justice Department spokeswoman Kerri Kupec.

“In a cordial and professional conversation, the special counsel emphasized that nothing in the attorney general’s March 24 letter was inaccurate or misleading,” Kupec said in a statement. “But, he expressed frustration over the lack of context and the resulting media coverage regarding the special counsel’s obstruction analysis.”

Mueller sought to convince the attorney general to release portions of the report so the public could understand the special counsel’s reasoning. But Barr declined, saying it would be better to release the entire redacted report, not put it out in piecemeal fashion.

Full coverage: Robert Mueller finishes his investigation into President Trump’s 2016 campaign »

In Barr’s earlier letter to Congress, he said Mueller “did not find that the Trump campaign, or anyone associated with it, conspired or coordinated with the Russian government in” its efforts to stoke division over social media or to hack Democratic computers and release troves of internal documents and emails.

He also wrote that Mueller’s report did not conclude that the president had committed a crime related to obstructing federal investigators but it also hadn’t exonerated him of those allegations.

However, Barr wrote, he had made a determination “that the evidence developed during the special counsel’s investigation is not sufficient to establish that the president committed an obstruction-of-justice offense.”

The letter did not disclose that Mueller had concluded that the Trump campaign “expected to benefit” from the emails stolen from Democrats. The special counsel also laid out “substantial evidence” of multiple instances when Trump acted with the intent of constraining or undermining the investigation then underway.

In his report, Mueller did not conclude whether Trump had committed the crime of obstructing justice, noting that Justice Department guidelines bar sitting presidents from facing charges. However, he wrote, the special counsel would not be precluded from saying Trump was innocent of such allegations.

“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,” he wrote. “Based on the facts and the applicable legal standards, however, we are unable to reach that judgment.”

At a news conference on April 18, about 90 minutes before he released the redacted report, Barr repeatedly said Mueller found no “collusion” between Trump associates and Moscow, repeating a favorite line of the president, even though Mueller expressly rejected using such language in his report.

The attorney general also sought to put the president’s conduct “in context,” saying, “there is substantial evidence to show that the president was frustrated and angered by a sincere belief that the investigation was undermining his presidency” without proof of wrongdoing.

“This evidence of non-corrupt motives weighs heavily against any allegation that the president had a corrupt intent to obstruct the investigation,” Barr said.

While Democrats are sure to probe Barr on Wednesday over his characterizations of Mueller’s work, Republicans are expected to question Barr about the origins of the Russia investigation in 2016 and whether it was politically motivated from the start, as Trump and his supporters describe it.

Barr has said he will review the court-authorized surveillance of a former Trump campaign aide during the 2016 campaign, which Barr described as “spying” during a previous hearing. The Justice Department’s inspector general is also reviewing the matter.

Barr also is scheduled to appear Thursday before the House Judiciary Committee, which is controlled by Democrats. But he has resisted plans by Rep. Jerrold Nadler (D-N.Y.), the panel’s chairman, to have lawyers for each party ask some of the questions.

If Barr refuses to show up voluntarily, Democrats could issue a subpoena or even hold him in contempt. The potential skirmish would escalate the growing battle between the White House and House Democrats pursuing an array of investigations on Capitol Hill.

Mueller has not spoken publicly since the 448-page redacted report was released, although Democrats have asked him to testify this month.

In a tweet Tuesday night, Senate Minority Leader Charles E. Schumer wrote that the “misleading nature” of Barr’s testimony and news conference “is even more glaring.”

“Barr must bring the letter with him when he testifies in the Senate,” Schumer added. “And it’s time for Mueller to testify publicly. Now.”