Before wrapping up his probe, Mueller helped secure guilty pleas from five people involved in Trump's presidential campaign - including Paul Manafort, who was his campaign chairman, and Michael Flynn, who became his first national security adviser. He's also indicted more than two dozen Russian hackers and military intelligence officers. President Donald Trump routinely rages against the investigation as a "witch hunt." Credit:AP While Mueller never said a word publicly, he and his team of prosecutors used indictments to set out a vivid narrative. It told of hackers tied to Russian intelligence agencies who stole Democratic emails to hurt Trump's opponent Hillary Clinton and who used social media to help spawn division with false and racially charged messages. It uncovered revealing Russian contacts with Trump's inner circle, such as a meeting in 2016 where Manafort shared polling data with a fixer tied to Russian intelligence. Loading But the full extent of what Mueller learned hasn't been revealed - and may not be if he or Barr decide to withhold details that the special counsel didn't feel involved crimes he felt he could prosecute.

Mueller's decision to issue a final report indicates that he chose not to indict other major figures in his investigation, including members of Trump's family and the president. However, if he secured any indictments under seal, they could be handed off to other elements of the Justice Department, such as a US attorney's office. Loading Replay Replay video Play video Play video Mueller's report could be politically disastrous for Trump if the special counsel says he uncovered evidence that would justify a congressional move to impeach the president - and it's sure to be claimed as vindication by the president if he doesn't. Even so, Trump isn't necessarily in the clear. He also faces continuing risk from other investigations, with federal prosecutors in New York looking into his company, presidential campaign and inaugural committee. Mueller has been sharing some matters and handing off others to US attorney's offices in Manhattan; Alexandria, Virginia; and Washington, as well as the Justice Department's national security division, giving cases that touch on his personal and business affairs a longer lease on life. Nor is it certain that others close to the president - including Donald Trump Jr., who met with a Russian lawyer after being promised dirt on Clinton - are out of jeopardy. Other prosecutors may well be pursuing investigations related to them.

Barr's Decision During Barr's confirmation hearing in February, he said Mueller's report would be confidential while "the report that goes public would be a report by the attorney general." He suggested that he might exclude criticism of Trump as inappropriate for any such public report because Justice Department guidelines argue against indicting a sitting president. US Attorney-General William Barr has suggested that he might exclude criticism of Trump as inappropriate for a public report. Credit:AP "If you're not going to indict someone, then you don't stand up there and unload negative information about the person," he said.

The Justice Department probably won't want to release the names of people that Mueller investigated but didn't charge. Material related to ongoing law enforcement operations, grand jury proceedings or classified intelligence programs is also expected to be withheld from the public. Ultimately, the Supreme Court may decide the fate of Mueller's findings. Trump and his lawyers have indicated they want the opportunity to issue a rebuttal on anything damaging to the president and to assert executive privilege over any disclosures of his actions during the presidential transition and the presidency. Loading But congressional Democrats - who now control the House - say they want broad disclosure of Mueller's investigative work, citing the earlier success of Republicans in pressuring the Justice Department to release details they said showed anti-Trump bias in the FBI. They have talked of issuing subpoenas to force disclosure and even public testimony by Mueller. "We're going to insist on the underlying evidence," Representative Adam Schiff, the Democratic chairman of the House Intelligence Committee, said in February on ABC's This Week.

"If you take the position that the president cannot be indicted, and the only remedy for improper, illegal or other conduct is impeachment, then you cannot withhold that information from Congress, or essentially the president has immunity," he said. Trump told reporters on Wednesday that he, too, wants Mueller's report made public. "Let it come out," he said. "Let people see it." Collusion, Obstruction Mueller, a former FBI director, was appointed in May 2017 to conduct one of the most consequential investigations in US history.

Beyond Russia's election meddling - which US intelligence agencies found was aimed at hurting Clinton and ultimately at helping Trump win - Mueller has been probing possible collusion in the operation and whether Trump sought to obstruct justice. In particular, Mueller investigated Trump's efforts to get then-FBI Director James Comey to drop an investigation into his first national security adviser, Michael Flynn. Mueller also investigated whether Trump's decision to fire Comey in May 2017 constituted obstruction of justice. Deputy Attorney General Rod Rosenstein appointed Mueller as special counsel days after Comey's firing. While Trump has often tweeted that "NO COLLUSION!" with Russia has been found, Mueller's inquiry has resulted in indictments of figures including the president's longtime adviser Roger Stone. Roger Stone, former campaign adviser for President Donald Trump. Credit:AP

Mueller indicted and convicted Manafort, Trump's former campaign chairman, for a series of financial crimes, and Manafort has been sentenced to 7½ years in prison. He also secured guilty pleas and cooperation agreements from Flynn and Trump's deputy campaign chairman Richard Gates and he worked with federal prosecutors in New York who secured a deal with former lawyer and fixer Michael Cohen. Mueller's investigation cost about $US25 million ($35 million) from his appointment in May 2017 through September 2018, according to the latest figures, provided by the Justice Department in December. It's been a fast-paced project compared to other major investigations of sitting presidents. Independent Counsel Kenneth Starr spent four years investigating President Bill Clinton before releasing his 1998 report on the Monica Lewinsky affair, which spun out of a probe into an Arkansas land deal known as Whitewater. The length and cost of that inquiry - and Starr's public release of a report with sexually explicit details about Clinton's relations with Lewinsky, an intern - contributed to Congress letting the law authorising independent counsels expire. Mueller was named under a less expansive Justice Department regulation providing for special counsels.