Questions are mounting over special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s inquiry into whether President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE obstructed justice as lawmakers on Capitol Hill await the release of his report.

While Barr’s four-page letter to Congress on Sunday silenced suspicions Mueller would charge Trump or members of his campaign with conspiring with the Russian government, its contents only amplified the mystery surrounding the obstruction inquiry.

It remains unclear why Mueller declined to make a decision one way or another on whether Trump impeded his investigation, and Democrats have grown increasingly skeptical of Attorney General William Barr Bill BarrProsecutor says no charges in Michigan toilet voting display Judge rules Snowden to give up millions from book, speeches The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE’s judgment that the evidence was insufficient to accuse Trump of obstruction. They also argue he is not a neutral arbiter.

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Lawmakers are unlikely to get answers for weeks, as the Justice Department combs through Mueller’s 300-plus-page report to determine what can be publicly released.

Barr told Congress on Friday that he expects to have the report prepared for public release by mid-April, after officials scrub it of grand jury material, sensitive national security information and details that could impact ongoing investigations.

In the meantime, Democrats are focused on a lengthy memo Barr penned last year criticizing the obstruction inquiry and labeling the theory Trump impeded the probe by firing FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE "fatally misconceived."

Some legal experts have described Mueller’s decision to not make his own call on obstruction charges as unusual. Regulations governing Mueller’s appointment required him to submit a report to Barr laying out why he prosecuted or declined to prosecute certain crimes.

"It is a little unusual because there is a charging decision, and either someone is charged or the prosecution is declined — a declination. It’s more unusual for a prosecutor to say, I don’t know or I don’t know enough to reach a firm conclusion," said Jack Sharman, a defense attorney at Lightfoot, Franklin & White and a former special counsel to Congress during the Whitewater investigation.

Some say it’s possible that Mueller meant to lay out the facts and let Congress decide on whether Trump obstructed the investigation or that he meant for Barr, a political appointee, to ultimately make the call.

Mueller was also keenly aware of the Justice Department policy not to indict a sitting president, which may have affected his reasoning.

Some argue Barr overstepped his bounds by making his own judgment on obstruction without releasing Mueller’s report or the evidence backing it up.

Others say it was Barr’s call to make given Mueller’s decision to not make a decision.

"It doesn’t say what happens if Mueller says, I’m unable to make a determination," Steven Cash, a lawyer at Day Pitney and former counsel to Sen. Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (D-Calif.), said of Mueller’s original mandate.

Broadly, legal experts agree that proving obstruction of justice is difficult; it requires establishing that the offender acted to impede an official proceeding and that he or she acted with "corrupt intent."

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Doing so would almost certainly require Mueller to interview Trump in order to get to the bottom of his reasons for firing Comey or taking other actions that the special counsel likely examined, such as his tweets attacking then-attorney general Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE and calling on him to end Mueller’s probe.

Mueller never reached an agreement with Trump’s personal lawyers on an interview and ultimately did not pursue a subpoena to compel his testimony. Why he did not subpoena Trump is another mystery.

"The fact that we ended up right on a tightrope on whether or not it was obstruction underscores how important it was for Mueller to interview or subpoena Trump," said Elie Honig, a defense attorney at Lowenstein Sandler and a former federal prosecutor.

Barr’s letter, quoting from Mueller’s report, notes that Mueller recognized "the evidence does not establish that the President was involved in an underlying crime related to Russian election interference" and suggests that makes it more difficult to prove the president was deliberately improper in his actions.

There is also debate as to whether a president can be accused of obstructing justice; some argue that Trump has the constitutional authority to remove Comey and order the Justice Department who and who not to investigate.

House Democrats are pushing for the full and immediate release of Mueller’s report as well as the underlying evidence, saying they cannot assess the findings until they see all the details the special counsel collected in the course of his 22-month investigation. Republicans, including Trump, are also in favor of the release of Mueller’s report.

Democrats have pointed to the June memo that Barr, a former attorney general under George H.W. Bush, penned to the Justice Department and White House, suggesting that he was biased in his handling of Mueller’s report, particularly on the question of obstruction.

In that memo, which was reported on soon after Trump nominated him for attorney general, Barr argued that Mueller’s investigation into Trump’s firing of Comey was "premised on a novel and legally insupportable reading of the law" and "would do lasting damage to the Presidency and to the administration of law within the Executive branch."

Democrats say they want to hear from Mueller, not Barr.

"We want to be clear, and I think that this letter that Barr wrote last Sunday underscores Congress's view that we do not want anything in the words of the attorney general. We want to see Robert Mueller's words. That's very critical," a House Democratic aide told reporters at a meeting in the Capitol on Thursday.

Democrats are demanding that Barr meet an April 2 deadline to produce Mueller’s full report to Congress. Barr on Friday said Justice would have the report ready for public release by mid-April or sooner.

Barr also told Congress on Friday that he would not provide the White House an advanced copy of the report to review for issues of executive privilege.

"Everyone will soon be able to read it on their own," Barr wrote. "I do not believe it would be in the public’s interest for me to attempt to summarize the full report or to release it in serial or piecemeal fashion."

Mike Lillis contributed.