7 key questions and answers about Mueller's report on Trump and Russia

Shortly after Attorney General William Barr sent a letter to Congress on Sunday laying out the "principal conclusions” of special counsel Robert Mueller’s investigation, the top two Democratic leaders on Capitol Hill issued a statement saying Barr’s letter "raises as many questions as it answers."

Indeed, ABC News reporters covering the Mueller report have been bombarded with questions about how Mueller’s findings came to be and what it will all mean.

Here are seven key questions and the best answers available:

Did Mueller find any evidence of obstruction of justice?

Mueller did find at least some evidence pointing to possible obstruction by President Donald Trump, and Mueller’s report "sets out evidence on both sides of the question," including evidence not in public view, according to Barr.

In fact, Mueller wrote in his report that "while this report does not conclude that the President committed a crime, it also does not exonerate him."

Barr, however, said he and Deputy Attorney General Rod Rosenstein ultimately decided there wasn’t "sufficient evidence to establish" a crime had been committed, especially because – Barr said – Trump’s actions didn’t "constitute obstructive conduct" or have "corrupt intent."

According to federal guidelines, the Justice Department should file charges not when there’s just evidence of criminal conduct, but when there’s enough evidence for prosecutors to "reasonably expect" a conviction.

"[N]o prosecution should be initiated against any person unless the attorney for the government believes that the admissible evidence is sufficient to obtain and sustain a guilty verdict," reads the Justice Department’s manual for federal prosecutors.

Did a years-old Justice Department legal opinion shield Trump from charges?

In 2000, in the aftermath of the independent counsel’s investigation of then-president Bill Clinton, the Justice Department issued an opinion stating that, based on the Constitution and other legal findings, a president cannot be indicted while still in office.

To indict a sitting president would interfere with his constitutional responsibilities and authorities, according to the opinion.

Ahead of Mueller’s final report, many Democrats worried that the Justice Department’s 2000 opinion could shield Trump from indictment even if Mueller found enough evidence to prove a crime.

But in his letter to Congress, Barr said he and Rosenstein determined Trump should not face obstruction-of-justice charges "without regard to" and “not based on” the 2000 opinion.

Nevertheless, when asked by ABC News whether the years-old opinion played a part in Mueller’s inability to reach a final conclusion on alleged obstruction, Mueller’s office declined comment.

On Sunday, the chairman of the House Judiciary Committee, Rep. Jerrold Nadler, D-New York, insisted his committee is "going to move forward" with its investigation of obstruction of justice and other "abuses of power."

Why did Barr, picked by Trump himself, get to decide whether charges against Trump were warranted?

Unlike the "independent counsel" who investigated Clinton and was supervised by a federal judge, Mueller was appointed under newer "special counsel" regulations that make him a part of the Justice Department – working under the attorney general like any federal prosecutor.

Those regulations say a special counsel has "the full power and independent authority to exercise all investigative and prosecutorial functions of any [U.S.] Attorney." But a U.S. attorney’s "authority is exercised under the supervision and direction of the Attorney General and his/her delegates," federal regulations state.

In Mueller’s case, the special counsel declined to "draw a conclusion" on whether the evidence he gathered warranted obstruction-of-justice charges against Trump, which “leaves it to the Attorney General” to decide, Barr said in his letter.

Will Congress – and the public – get to see Mueller’s full report?

It’s unlikely that the entire Mueller report, with no redactions at all, will be released, especially because it includes grand jury material that is prohibited from public release. But in his letter to Congress, Barr said he is “mindful of the public interest in this matter” and intends “to release as much of the Special Counsel’s report as I can.”

He said he will have to consider "applicable law, regulations, and Departmental policies," including the impact on ongoing investigations, in determining what can be released.

It’s unclear how long that process may take. Democrats, meanwhile, have vowed to subpoena Mueller’s materials and even take the Justice Department to court if it comes to that.

Was Barr conflicted from the start?

During Barr's confirmation hearing to become attorney general, Democrats pressed him over a memo he sent the Trump administration last year arguing that "Mueller's obstruction theory is fatally misconceived."

Democrats wondered how Barr could fairly assess Mueller's evidence when he had already expressed doubt about the obstruction probe itself.

But Barr insisted to lawmakers that he was only arguing in the memo that certain events described in media accounts, such as the firing of James Comey as FBI director, did not themselves constitute obstruction due to the president’s inherent authorities.

"I realize that I am in the dark about many facts," he wrote in the June 2018 memo.

After Barr's Senate confirmation, "senior career ethics officials" reviewed the matter and determined Barr "should not recuse himself" from overseeing Mueller's probe, according to a recent statement from Justice Department spokeswoman Kerri Kupec.

Month earlier, however, senior career ethics officials reached a different conclusion about then-acting attorney general Matt Whitaker, telling Whitaker he should recuse himself from oversight of Mueller's probe for his previous comments about it. Whitaker declined to follow their advice but later told lawmakers he never interfered with the investigation.

On Sunday, a joint statement from top Democrats insisted Barr’s "unsolicited, open memorandum to the Department of Justice … [still] calls into question his objectivity."

What was Rosenstein’s role in all this?

For the first year-and-half of Mueller’s investigation, Rosenstein was the ultimate supervisor of the probe – the one who could have blocked Mueller from taking certain steps. In that time, Rosenstein oversaw the investigation because then-attorney general Jeff Sessions had recused himself from the probe, citing his prior work on Trump’s presidential campaign.

When Sessions resigned in November 2018 and another Trump administration official was put in charge of the Justice Department, Rosenstein was no longer the ultimate supervisor of the case, but he retained a significant role in supervising Mueller’s operation, a Justice Department official told ABC News.

At no time did Rosenstein – or any other Justice Department leader – block Mueller’s investigators from taking investigative actions they wanted to pursue, according to the Justice Department.

Rosenstein, however, was consulted by Barr over whether Mueller’s evidence warranted charged against Trump for obstruction of justice. Three weeks ago, Rosenstein and Barr met with Mueller’s team inside the Justice Department, where Rosenstein and Barr were given an overview of Mueller’s final conclusions, including his decision not to weigh in on whether Trump had committed obstruction of justice, according to a source familiar with the meeting.

Moving forward, Rosenstein will be consulted over what further information from Mueller’s investigation can be publicly released, Barr told Congress.

How long is Mueller's report?

This is still a secret, and it’s unclear whether the Justice Department will ever be willing to say how many pages comprise Mueller's final report.