On that key question, Mueller "ultimately determined not to make a traditional prosecutorial judgment," Barr wrote, leaving it up to the attorney-general and Deputy Attorney General Rod Rosenstein to decide whether the President had committed obstruction.
Rosenstein and Barr "concluded that the evidence developed during the Special Counsel's investigation is not sufficient to establish that the President committed an obstruction of justice offence. Our determination was made without regard to, and is not based on, the constitutional considerations that surround the indictment and criminal prosecution of a sitting president," Barr wrote.
Barr further explained that decision by writing "the report identifies no actions that, in our judgment, constitute obstructive conduct, had a nexus to a pending or contemplated proceeding, and were done with corrupt intent, each of which, under the Department's principles of federal prosecution guiding charging decisions, would need to be proven beyond a reasonable doubt to establish an obstruction of justice offence."
Mueller's central mission has been to determine if Russian efforts to interfere in the 2016 election were aided or assisted in any way by Americans, including people close to Trump.
In all, Russian citizens interacted with at least 14 Trump associates during the campaign and presidential transition, according to public records and interviews.
Of particular concern was the interaction between a London-based professor and a low-level Trump foreign policy adviser, George Papadopoulos. According to court filings, the professor told Papadopoulos in April 2016 that the Russians held damaging information about Trump's opponent, Hillary Clinton, in the form of thousands of emails.
Mueller also dug into a June 2016 meeting at Trump Tower in New York. Trump's son Donald Trump jnr and Trump's son-in-law Jared Kushner met with a Russian lawyer after being told she had incriminating information on Clinton that was being offered as part of the Russian government's support for the GOP candidate, according to emails exchanged in advance of the meeting.
The lawyer has said she was not working on behalf of the Russian Government. Trump jnr and Kushner have said she did not provide any information about Clinton at the meeting.
Seeking to answer the collusion question, Mueller has also scrutinised the anti-secrecy group WikiLeaks, which released batches of Democrats' emails that US investigators say were stolen by Russian intelligence officers.
Since his appointment in May 2017, Mueller has also wrestled with the question of whether the President attempted to obstruct justice once the FBI began investigating those close to him.
Current and former White House officials who were questioned by Mueller's investigators were repeatedly asked about how the President spoke of the investigation behind closed doors, and whether he sought to replace senior Justice Department officials to stymie the probe, according to people familiar with the interviews.
The Special Counsel's work led to criminal charges against 34 people, including six former Trump associates and advisers.
Yesterday, officials said that one of those cases — that of Trump's former deputy campaign chairman Rick Gates — will be transferred from the Special Counsel's office to federal prosecutors in Washington. Gates pleaded guilty last year to conspiracy and lying to the FBI, and he continues to cooperate with prosecutors while awaiting sentencing.
A senior Justice Department official said the Special Counsel has not recommended any further indictments — a revelation that buoyed Trump's supporters, even as additional Trump-related investigations continue in other parts of the Justice Department, in Congress and in New York state.
