Updated April 2 at 4:52 p.m. ET
Special counsel Robert Mueller's report has been a long time in the making.
Mueller was appointed in the spring of 2017 by Deputy Attorney General Rod Rosenstein after President Trump fired FBI Director James Comey.
Mueller, a former FBI director who had joined a top law firm after leaving government service, was commissioned to investigate:
"any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump; and any matters that arose or may arise directly from the investigation."
Comey confirmed earlier in 2017 that the FBI had opened an investigation into whether anyone on Trump's successful 2016 presidential campaign worked with the Russians who had attacked the election.
Mueller then took over with a team of prosecutors, investigators and other specialists brought in from the Justice Department, the intelligence community and the private sector.
A hall-of-mirrors atmosphere has reigned in Washington ever since as the capital waited on tenterhooks to see what Mueller would uncover — and what that might mean politically for Trump.
Old roles were inverted as Republicans tore into federal law enforcement and complained about conspiracies and bias within a "deep state" they said was trying to frame Trump.
Comey complained that the president and his allies attempted no less than to "burn down the entire FBI" in order to deflect attention from themselves.
Democrats, meanwhile, took up the mantle of law and order, championing the FBI and the intelligence community; Mueller himself became a cult figure for Trump opponents who lit prayer candles, silk-screened T-shirts that said "It's Mueller Time" or waited until their last breath for his report to be completed.
Trump went back and forth as to what he accepted about the underlying interference by Russia in 2016, but he maintained throughout that neither he nor anyone in his campaign had anything to do with it.
The concept of collusion was a "hoax," and the investigation into what took place during the political cycle that elected him was a "witch hunt," Trump has said repeatedly.
The president blamed what he called Democrats angry about the loss of an election in 2016 they believed they should have won. He and supporters also said they suspected a conspiracy within federal law enforcement that was out to get him.
Investigating the investigators
Even as the Justice Department became a target of outsiders trying to help the president, its internal investigators found big problems of their own.
The deputy FBI director, Andrew McCabe, was fired after he was found to have "lacked candor" in talking about his dealings with the press in 2016.
During multiple interviews in support of a new book, McCabe said he intends to sue the Trump administration for wrongful termination, among other things, because, in McCabe's view, he was singled out and personally targeted for his work on the FBI investigation.
Investigators also discovered that a senior FBI attorney, Lisa Page, and a senior counterintelligence agent, Peter Strzok, exchanged a number of text messages on their work phones that were critical of Trump in 2016 as they worked on some of the bureau's most politically sensitive cases.
Strzok had been involved with the Russia investigation and moved for a time into the special counsel's office — until the messages were discovered and he was removed from Mueller's team.
The anti-Trump sentiment in the messages embarrassed the FBI and opened it up to months' worth of criticism about its fairness in the investigation. Some other DOJ or FBI officials caught up in the Russia imbroglio also were targeted or stepped down.
The president's GOP allies in Congress also sought more information from the Justice Department and FBI about the earlier phases of the investigation, before Mueller, arguing that it, too, showed bias or abuses of power.
Rosenstein fought a rearguard action against the then-Republican leadership of the House Judiciary Committee, which demanded sensitive information about surveillance practices and the use of confidential informants during the 2016 phase of the inquiry.
The dossier
Hanging above it all were questions about the now-infamous, unverified Russia dossier that played a role in at least one warrant obtained by FBI investigators to collect the communications of Carter Page, a 2016 Trump campaign foreign policy adviser.
The FBI and the Justice Department treated the material in the dossier as trustworthy enough to pursue their own inquiries about it behind the scenes, but it became internationally notorious after an unredacted copy was published in early 2017.
NPR has not detailed the contents of the dossier because it was and remains unverified, but the document became the centerpiece of a political war because of the explosive allegations it makes about Trump and his 2016 campaign.
Members of Congress attacked it and defended it, but only a few points about it have been established for certain. One is that investigators have verified only portions of what it says — although precisely which portions remains classified.
Another is that the dossier wasn't the origin of the Russia investigation. By the time it came along, the FBI already had begun looking into reports from an Australian diplomat in London who said an American on Trump's campaign had told him the Russians had promised dirt on Hillary Clinton.
That led to the guilty plea by George Papadopoulos, a junior Trump campaign foreign policy aide, and inquiries into other members of the Trump camp, including campaign chairman Paul Manafort, national security adviser Michael Flynn and another junior foreign policy adviser, Carter Page.
Papadopoulos, Manafort and Flynn all were convicted or have pleaded guilty in the course of the Russia investigation; Page so far has not faced any charge.
The black box
Through it all, however, Mueller and his staff did not get caught up in partisan warfare themselves — they gave no interviews, convened no press conferences and kept almost all of their own secrets.
Mueller's office made less than a handful of public statements of any kind. Once, it defended the practices it was using to secure evidence. Once, it said it was referring an alleged scheme to discredit Mueller to the main FBI.
And once it disputed a report that said it had documents that said Trump had instructed his onetime attorney Michael Cohen to lie to Congress.
Otherwise the special counsel's office was a black box about which Washington endlessly speculated, commentated and fulminated.
That meant that Rosenstein often was the public face of the Russia investigation, not Mueller. And it meant that the workings and intentions of the special counsel's office largely remained a cipher — until the completion of its final report and its expected release to the public and Congress.
Attorney General William Barr notified lawmakers about his receipt of the report on March 22.
He summarized two of its most politically important findings: That Mueller did not establish that Trump's 2016 campaign conspired with the Russians who attacked the election and that Mueller didn't reach a firm conclusion about whether Trump obstructed justice.
Barr and Rosenstein, the attorney general wrote, determined themselves that Mueller's investigation wasn't sufficient to establish that Trump had committed an obstruction offense.
Barr's office then began work on redacting Mueller's nearly 400-page report in order to protect grand jury information, foreign intelligence and other sensitive material, in preparation for the document to Congress.
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