He's been the subject of alt-right conspiracy theories, labelled an "errand boy" for Hillary Clinton, even accused of being a leftist spy.

Australian diplomat Alexander Downer's warning to his US counterpart in London was the "tipping point" for an FBI probe into Russia's interference in the 2016 US election and, even now, may form part of impeachment proceedings against President Donald Trump.

Now, thanks to the release of a watchdog report, we know what he said.

First, some background

It started with a night out at the Kensington Wine Room, a posh bar in London, in May 2016.

As Australia's High Commissioner to the UK, Mr Downer was meeting with a member of Mr Trump's campaign team, George Papadopoulos.

According to Mr Downer's account of the night, Papadopoulos revealed information about Russia's plans to interfere in the US election before the release of tens of thousands of emails authored by Mr Trump's opponent Hillary Clinton, a claim Papadopoulos has denied.

"[Papadopoulos] said one of the reasons [Trump would win] was that the Russians might release some information which could be damaging to Hillary Clinton," Mr Downer said previously.

In a diplomatic cable to Canberra, Mr Downer downplayed the significance of Papadopoulos's apparent prediction.

But when WikiLeaks subsequently dumped the Clinton emails, Mr Downer requested a meeting about an "urgent matter" with his counterpart at the US embassy in London.

Now we know what he said

A report by US Department of Justice inspector-general Michael Horowitz has today revealed exactly what the then-high commissioner said.

Referred to in the report only as a "friendly foreign government official", Mr Downer said that Papadopoulos "suggested the Trump team had received some kind of suggestion from Russia that it could assist this process with the anonymous release of information during the campaign that would be damaging to Mrs Clinton (and President Obama)."

Deputy FBI director Andrew McCabe characterised Mr Downer's statement as a "tipping point" in the July 2016 decision to open an investigation into Russia's attempts to interfere with the 2016 election.

"Because not only was there information that Russia was targeting US political institutions," the report states.

"Now the FBI had received an allegation from a trusted partner that there had been some sort of contact between the Russians and the Trump campaign."

So why does it matter?

The FBI's probe led to US special counsel Robert Mueller's two-year investigation into election meddling, which congressional Democrats may now use as evidence that Mr Trump obstructed justice as they start drawing up articles of impeachment.

In short, Mr Downer's suggestion that the Trump campaign "received some kind of suggestion from Russia" set off a chain of events that, along with more recent claims about Ukraine, are likely to see Mr Trump impeached by the Democrat-controlled House of Representatives and tried in the Senate.

It's no wonder Trump allies have gone after the former Australian foreign minister.

What are the conspiracy theories?

Many of the theories that spread in right-wing chatrooms originated with Papadopoulos himself. He has repeatedly claimed, without evidence, that Mr Downer recorded their now-infamous wine bar meeting on his smartphone.

Papadopoulos and other Trump supporters often cite a theory that the intelligence services of several countries, including Australia, had a secret plan to disrupt Mr Trump's campaign.

Loading

Loading

Mr Papadopoulos served a 14-day prison sentence last year after admitting he had lied in a 2017 interview with the FBI, hindering their investigation.

The wild theories have made it all the way into the halls of power.

Republican senator Lindsey Graham, a staunch Trump supporter, gave credence to the tale, suggesting in October that Mr Downer was "directed" to seek a meeting with Papadopoulos.

He wrote that "US intelligence communities" accepted "information from an Australian diplomat who was also directed to contact Papadopoulos and relay information … to the Federal Bureau of Investigation".

In a letter to Senator Graham, published on Twitter, US ambassador Joe Hockey rejected the assertion.

Loading

There were also claims that by reporting the matter directly to the US embassy in London, Mr Downer did not go through the correct diplomatic channels.

Mr Graham even implied Australia might be working against Mr Trump by initially refusing to release the text of Mr Downer's report.

Australia probably didn't act improperly … but the FBI did

Today's report offered a 476-page deep-dive on the origins of the Mueller probe. Mr Downer's role as the "friendly foreign government" (FFG) official is scattered throughout, but the main focus is the FBI's propriety.

There's no criticism of Mr Downer or Australia's role, and it's conceivable that, had the inspector-general discovered wrongdoing, he would have found the space to mention it in such a sweeping report.

US Attorney-General William Barr told The Australian that Mr Downer "did the right thing in supplying that information; the FFG has acted at all times just as we would hope a close ally would".

"We are grateful that we have such friends," he said. "What was subsequently done with that information by the FBI presents a separate question."

Only time will tell whether the Australia/Downer conspiracy theories will fizzle out, starved of fuel.

In the meantime, Mr Trump's allies may find alternative sources of ammunition in the report's pages.

The inspector-general ultimately "did not find any documentary or testimonial evidence that political bias or improper motivation influenced the FBI's decision to conduct these operations," which contradicts Mr Trump's "witch hunt" narrative.

However, a substantial portion of the report criticises the FBI for failing to meet its own standards of accuracy and completeness in filing applications for surveillance into a member of the Trump campaign.

One low-ranking FBI lawyer may even face prosecution for altering a document related to FBI wiretaps.

Judging by early reactions, this won't be the last we hear about it

Mr Trump described the report's findings as "a disgrace".

"It's far worse than I would've ever thought possible. It's an embarrassment to our country. It's dishonest."

Loading

Loading

Loading

A separate review is also underway, led by prosecutor John Durham, who was handpicked by the President's ally, Mr Barr.

In a rare statement, Mr Durham publicly said he disagreed with today's report, as did the Attorney-General.

Loading

The President says he is waiting for Mr Durham's report.