It is one of the great mysteries of Donald Trump’s presidency and the answer, if exposed at a high-stakes US congressional hearing this week, could rock the US-Australian alliance.

How did highly sensitive information divulged at a London wine bar in May 2016, by Mr Trump’s then campaign adviser George Papadopoulos to Australia’s then UK High Commissioner Alexander Downer, make its way to the US intelligence community?

The Papadopoulos-Downer meeting has been credited by the New York Times as being the seed that led to the initial FBI investigation into Russian interference in the 2016 presidential election and then Special Counsel Bob Mueller’s full-blown probe.

Mr Trump has repeatedly slammed the Mueller investigation as a “witch hunt”.

The clearest explanation of Mr Downer’s and the Australian government’s involvement could be revealed in a series of upcoming public congressional hearings in Washington DC.

The first is scheduled for Thursday (Friday AEST) when controversy-plagued FBI counterintelligence agent Peter Strzok testifies under oath before the House Judiciary Committee.

Devin Nunes, chairman of the US House Intelligence Committee, is also requesting Elizabeth Dibble, former deputy chief of mission at the US embassy in London, and others be interviewed by the Judiciary and Oversight & Government Reform committees.

“For the sake of transparency and to keep the American people as fully informed as possible about these matters, the task force should consider interviewing these individuals in an open sitting,” Mr Nunes wrote in the letter to committee chairman Trey Gowdy and Bob Goodlatte.

Mr Papadopoulos reportedly told Mr Downer, months before Mr Trump’s election victory, Russia had political dirt on then presidential frontrunner Hillary Clinton.

The New York Times, Wall Street Journal and other leading publications, often citing unnamed sources, have published multiple reports on the Papadopoulos-Downer meeting, but the timeline and route of how the information shared at the Kensington Wine Rooms made its way to the US intelligence community remains murky.

The New York Times, in a story titled How the Russia Inquiry Began: A Campaign Aide, Drinks and Talk of Political Dirt described how “Australian officials passed the information about Mr Papadopoulos to their American counterparts” two months after the meeting when leaked Democratic Party emails began appearing online.

The Australian newspaper interviewed Mr Downer in April about the Papadopoulos meeting and reported “within 48 hours Downer had sent an official cable about what he had heard to Canberra” and “after a period of time, Australia’s ambassador to the US, Joe Hockey, passed the information on to Washington”.

A Wall Street Journal columnist disputes that.

“A diplomatic source tells me Mr Hockey neither transmitted any information to the FBI nor was approached by the US about the tip,” WSJ columnist Kimberley Strassel wrote.

“Rather, it was Mr Downer who at some point decided to convey his information – to the US embassy in London.”

If Mr Downer did go directly to the US embassy, that could be seen as a major breach in diplomatic protocol.

Mr Strzok, who has come under fire for anti-Trump text messages he sent to a girlfriend, also an FBI agent, is a key player because he reportedly flew to London to interview Mr Downer early in the FBI probe.

If Ms Dibble testifies on Capitol Hill, she is expected to be grilled about her contact with Mr Downer.

Australian Prime Minister Malcolm Turnbull and the Mr Trump have worked hard to mend their relationship after a contentious January 28, 2017 phone call about asylum-seekers a week after Mr Trump moved into the White House.

The leaders have met at various choreographed events since and Australia created the 100 Years of Mateship program.

If the “witch hunt” was born out of Mr Downer’s London drinks, Australia may again find itself in the crosshairs of Mr Trump’s fury. 

-AAP