Special counsel Robert Mueller's investigation into Donald Trump's campaign for President and its connections to Russia has become so broad and complex that people following it closely are running out of red string and space on their giant conspiracy walls.

But all conspiracy walls have focal points. Spots where heaps of red string all tie together into a big wad. Significant moments which changed the course of the alleged conspiracy, and investigation.

Here are eight of the big ones to keep an eye on when Mr Mueller's report is released.

The letter of intent

October 13, 2015

Donald Trump's business, the Trump Organisation, has undergone significant changes since its heyday in the 1980s.

After multiple bankruptcies in the early '90s, cash flow became a problem for Trump's property development empire and he was blacklisted by major banks.

This led Trump to restructure his business into primarily a licencing company, where other companies pay him a fee to use his name on their buildings and products.

It meant he was doing business with some unsavoury and questionable figures.

While this was enough to keep Trump afloat, it wasn't hugely lucrative, and he was on the lookout for more big deals.

In 2015, Trump came close to closing the biggest deal of his life: a deal with Russian banks, developers and the Russian government to build a Trump Tower in Moscow which would bring hundreds of millions of dollars his way.

Felix Sater, centre, with Donald and Melania Trump in an undated photograph. ( Supplied: Felix Sater )

Trump's business associate Felix Sater emailed Trump Organisation lawyer Michael Cohen on October 13, 2015, with a letter of intent signed by a Russian developer for Donald Trump to sign, taking steps toward building the tower.

Cohen has now admitted he lied to Congress about the project when he said it had been abandoned early in the presidential campaign.



The wine bar

May 10, 2016

George Papadopoulos was very excited when he secured a position on presidential candidate Donald Trump's foreign policy advisory team in March 2016.

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His only qualifications for the job were some experience in the oil and gas industry and time working on another Republican presidential candidate's team, but after meeting with Trump and other senior campaign officials, he went to London and got to work.

In London, the 29-year-old was introduced to a mysterious figure named Joseph Mifsud, a professor at Scotland's University of Stirling who apparently had high-level connections with the Russian government.

Through his communications with Mifsud, Papadopoulos was told that Russia had in its possession thousands of hacked Democratic emails which were damaging to Hillary Clinton.

In a conversation with Australian High Commissioner to the UK Alexander Downer at Kensington Wine Rooms in London, Papadopoulos mentioned the emails.

The Trump Tower meeting

June 9, 2016

As part of his decades-long campaign to crack into the Russian property market, Donald Trump held his 2013 Miss Universe pageant in Moscow.

During that venture, he made contact with Putin-connected billionaire property oligarch Aras Agalarov, his popstar son Emin, and Emin's publicist Rob Goldstone.

In early June 2016, Goldstone sent an email to Donald Trump, Jr, saying that the Russian government was interested in assisting the Trump campaign, and had incriminating documents and information about Hillary Clinton they wanted to share.

Don Jr responded in an email saying, "If it's what you say I love it especially later in the summer."

A meeting was set up between Don Jr, a Russian government-connected lawyer, a Russian-American lobbyist with connections in the intelligence community, Goldstone, a translator and a representative of the Agalarov company.

Don Jr asked his brother-in-law turned campaign advisor Jared Kushner and Trump campaign manager Paul Manafort to attend the meeting as well.

The following day, Aras Agalarov delivered an expensive painting to Trump as a gift.

It's unclear exactly what happened in the meeting because when the media found out about it, everyone but Rob Goldstone made comments that later turned out to be lies.

Trump himself dictated a misleading statement on behalf of his son saying the meeting was about "Russian adoptions", when in fact it was about dirt on Hillary Clinton.

The reasons behind the Trump family's decision to lie about the meeting remain unclear.

Democrats pull the fire alarm

June 14, 2016

The Democratic National Committee publicly revealed that its system had been hacked, and emails and documents had been stolen, in June 2016.

Soon afterwards, state-sponsored Russian hackers released emails stolen from the Democratic Party, damaging Hillary Clinton's campaign.

The revelation of the hack led to two subsequent events:

The Trump Tower Moscow project was abandoned by the Trump Organisation

The Trump Tower Moscow project was abandoned by the Trump Organisation Australian diplomat Alexander Downer reported his conversation with George Papadopoulos to US authorities, beginning the FBI investigation which would become the Mueller inquiry a year later

Grabbed

October 7, 2016

Long-time Trump associate and Republican political operative Roger Stone has a colourful Twitter feed.

A well-known conspiracy theorist, Stone regularly tweeted misleading or incorrect information about Hillary Clinton.

In August 2016, he started predicting upcoming trouble for Clinton campaign director John Podesta.

Donald Trump with Billy Bush in his now-infamous Access Hollywood appearance. ( Washington Post via Access Hollywood )

In the first week of October, he started predicting that document leaking website WikiLeaks was about to publish damaging information about Hillary Clinton.

On October 7, he was proven right.

Less than an hour after the publication of a 2005 Access Hollywood recording in which Donald Trump boasted about sexually assaulting women, WikiLeaks started dumping emails stolen from John Podesta by Russian hackers on its website.

This quickly tempered the outrage over the Access Hollywood tape, and led major media outlets to begin talking again about the years-long Hillary Clinton email controversy.

It also raised questions about how Stone seemingly had previous knowledge of the hack, and whether he was serving as a conduit between WikiLeaks and the Trump campaign, assisting them to co-ordinate the release of the hacked emails for maximum effect.

The Mueller investigation said in February it had evidence of communication between Roger Stone and WikiLeaks over the release of the hacked emails.

Calling the ambassador

December 29, 2016

When President-elect Donald Trump visited the White House two days after the 2016 election, President Barack Obama gave him a very specific warning that he should not hire Lieutenant General Michael Flynn in his administration.

Flynn had been fired by Obama as a senior intelligence official in 2014, and since then had become a fierce critic of American foreign policy.

He had also built connections inside Russian media and the Russian government, and was paid to sit next to Vladimir Putin at a gala dinner in Moscow celebrating the Russian media outlet RT in 2015.

Michael Flynn was hired as National Security Advisor by Donald Trump. ( AP: Carolyn Kaster )

By the end of 2016 Flynn was also working as an undeclared foreign agent, advocating for the government of Turkish President Recep Tayyip Erdogan in Washington DC.

The Trump campaign was told of this, both by the Democrats in Congress and by Flynn's own lawyer, but nonetheless hired him as National Security Advisor.

In late December 2016, Barack Obama announced new US sanctions on Russia in response to Russian efforts to influence the US Presidential campaign.

Flynn quickly called the Russian ambassador to the US Sergey Kislyak to tell him not to retaliate, because the Trump administration would be revoking those sanctions when it came into office.

The phone conversation was picked up by American intelligence, which only became a problem when Flynn started lying about the call, saying he hadn't discussed sanctions with the ambassador.

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The US Justice Department warned the Trump administration that Flynn may be vulnerable to Russian blackmail, given that the Russians held evidence that Flynn was lying to the public.

The Trump administration refused to fire Flynn for more than two weeks, despite warnings that he had been compromised by the Russians.

Flynn eventually resigned from the White House after the story was leaked.

The firing

May 9, 2017

When the story about Michael Flynn's call with the Russian ambassador came to the attention of the FBI, director James Comey opened an investigation.

At the time, it wasn't publicly known that Comey was also investigating Russia and the Trump campaign for potentially working together to influence the 2016 election.

According to Comey, Trump tried to get him to give up on investigating Flynn, and asked for his loyalty.

Comey refused.

Former FBI Director James Comey, centre. ( AP: Susan Walsh )

The investigation into the Trump campaign was publicly acknowledged in March, and by May, Comey was asking for more resources to handle the broadening probe.

On May 9, Comey was fired by Trump. Comey revealed he found out he had been fired from a TV news report.

Comey's firing caused public and political outrage.

In the Justice Department, it led to discussions about whether the Cabinet could be persuaded to remove Trump from office using the 25th Amendment to the US Constitution, because he was unfit to hold the job.

It also led to demands that a special counsel be appointed, separate from the FBI, to investigate the broadening Trump/Russia scandal.

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Those demands were met eight days after Comey was fired, with the appointment of his predecessor as FBI director, Robert Mueller, as special counsel.

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Comey's firing and subsequent testimony also led to allegations that the President was committing criminal obstruction of justice by interfering in the investigation into him.

This was compounded by the President telling NBC News that he was thinking about the Russia probe when he fired Comey.

The Mueller inquiry is expected to shed light on whether the President committed a crime by firing Comey, as well as other actions aimed at undermining the probe.

Stormy weather

January 12, 2018

Lurid stories about Donald Trump aren't hard to come by.

He spent the 1980s, '90s and 2000s cultivating a playboy image, and wasn't shy about discussing his sexual exploits in print, or on TV and radio.

In the dying days of the 2016 election campaign, it was a different story.

When Trump became aware that two women were alleging he had affairs with him in 2006, he asked his personal lawyer Michael Cohen to take care of it.

Cohen made six-figure payments to Playboy model Karen McDougall and adult film actress Stormy Daniels in exchange for their silence.

Michael Cohen, centre, worked as Donald Trump's personal lawyer. ( AP: Mary Altaffer )

On 12 January 2018, the Wall Street Journal published a bombshell report. Donald Trump had reportedly paid off an adult film star to keep her quiet.

At first this seemed like just another sex scandal. But gradually it morphed into possibly the most significant single news report of the year.

Investigators thought the payment may have constituted an illegal campaign donation, as it was money which changed hands to influence the campaign.

This led to a raid on Michael Cohen's home, office and apartment, and subsequently, Cohen deciding to become a witness for the prosecution.

It provided a door into Donald Trump's business history.

It means if any of the unsavoury or questionable figures Trump has done business with over the years put any illicit cash in his pocket, Robert Mueller had a way of finding out about it.

Big questions

Most investigations into political scandals begin with a central event, and go on to investigate whether public officials engaged in a cover-up.

The Mueller investigation has approached this backwards, by providing significant evidence of a cover-up without much information of what Trump and his associates are trying to hide.

Several Trump associates are set to serve prison time for lying to investigators, but it's still unclear whether anything they were trying to hide amounts to a conspiracy against the United States.