US special counsel Robert Mueller has concluded his investigation into Russian interference in the US presidential election and alleged collusion with associates of Donald Trump.

The Justice Department said Mr Mueller delivered his final report on Friday to Attorney General William Barr, who is reviewing it.

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Attorney General William Barr is reviewing Robert Mueller’s report (Jose Luis Magana/AP)

Responding to the release, the White House said the next steps are “up to Attorney General (William) Barr”.

White House press secretary Sarah Sanders said “we look forward to the process taking its course”.

She added: “The White House has not received or been briefed on the Special Counsel’s report.”

Leading Democrats Nancy Pelosi and Chuck Schumer said it is “imperative” to make the full report public.

The top congressional Democrats said: “The American people have a right to the truth.”

In a joint statement, they said Mr Barr must not give Mr Trump, his lawyers or staff any “sneak preview” of the findings or evidence.

“The White House must not be allowed to interfere in decisions about what parts of those findings or evidence are made public,” they said.

Mr Mueller’s report sets the stage for big public fights to come. The next steps are up to Mr Trump’s attorney general, to Congress and, in all likelihood, federal courts.

It is not clear how much of the report will become public or be provided to Congress. Mr Barr has said he will write his own report summarising Mr Mueller’s findings.

The nearly two-year probe has shadowed Mr Trump’s presidency and resulted in felony charges against 34 people including six people who served on Mr Trump’s campaign.

Mr Barr said he could release his account to Congress as soon as this weekend.

“I am reviewing the report and anticipate that I may be in a position to advise you of the special counsel’s principal conclusions as soon as this weekend,” Mr Barr said in his letter to the top Republicans and Democrats on the House and Senate Judiciary committees.

Mr Trump’s lawyers said they were “pleased” that Mr Mueller has delivered his report.

Rudy Giuliani and Jay Sekulow said: “We’re pleased that the Office of Special Counsel has delivered its report to the Attorney General pursuant to the regulations. Attorney General Barr will determine the appropriate next steps.”

With no details released at this point, it is not known whether Mr Mueller’s report answers the core questions of his investigation:

– Did Mr Trump’s campaign collude with the Kremlin to sway the 2016 presidential election in favour of the celebrity businessman?

– Did Mr Trump take steps later, including by firing his FBI director, to obstruct the probe?

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President Donald Trump has been dogged by the investigation (Carolyn Kaster/AP)

But the delivery of the report does mean the investigation has concluded without any public charges of a criminal conspiracy between the campaign and Russia, or of obstruction by the President.

It is unclear what steps Mr Mueller will take if he uncovered what he believes to be criminal wrongdoing by Mr Trump, in light of Justice Department legal opinions that have held that sitting presidents may not be indicted.

The mere delivery of a confidential report will set off immediate demands, including in the Democratic-led House, for full release of Mr Mueller’s findings.

Mr Barr has said he wants to make as much public as possible, and any efforts to withhold details will prompt a tussle between the Justice Department and lawmakers who may subpoena Mr Mueller and his investigators to testify before Congress.

Such a move by Democrats would likely be vigorously contested by the Trump administration.

The conclusion of Mr Mueller’s investigation does not remove legal peril for the president. Mr Trump faces a separate Justice Department investigation in New York into hush money payments during the campaign to two women who say they had sex with him years before the election.

He has also been implicated in a potential campaign finance violation by his former lawyer, Michael Cohen, who says Mr Trump asked him to arrange the transactions. Federal prosecutors, also in New York, have been investigating foreign contributions made to the president’s inaugural committee.

No matter the findings in Mr Mueller’s report, the investigation has already illuminated Russia’s assault on the American political system, painted the Trump campaign as eager to exploit the release of hacked Democratic emails and exposed lies by Trump aides aimed at covering up their Russia-related contacts.

The special counsel brought a sweeping indictment accusing Russian military intelligence officers of hacking Democrat Hillary Clinton’s campaign and other Democratic groups during the 2016 election.

He charged another group of Russians with carrying out a large-scale social media disinformation campaign against the American political process that also sought to help Mr Trump and hurt Mrs Clinton.

Closer to the president, Mr Mueller secured convictions against a campaign chairman who cheated banks and dodged his taxes, a national security adviser who lied about his Russian contacts and a campaign aide who misled the FBI about his knowledge of stolen emails.

Cohen, the president’s former lawyer, pleaded guilty in New York to campaign finance violations arising from the hush money payments and in the Mueller probe to lying to Congress about a Moscow real estate deal.

Another Trump confidant, Roger Stone, is awaiting trial on charges that he lied about his pursuit of Russian-hacked emails ultimately released by WikiLeaks.

It is unclear whether any of the aides who have been convicted, all of whom have pleaded guilty and cooperated with the investigators, might angle for a pardon. Mr Trump has left open the idea of pardons.