One year after launching his investigation into Donald Trump's campaign links with Russia, Special Counsel Robert Mueller has rocked the team that helped carry the billionaire to the White House.

Mueller, a former FBI director, has maintained absolute silence about what his team of investigators are up to.

Here is what is known about the probe so far - as it spills into a second year - based on accounts of witnesses, lawyers, court filings and leaks.

Scores interviewed

Mueller's large team of investigators likely has interviewed more than 100 people from the campaign, government, and private industry, including foreigners.

READ MORE Trump furious over 'witch-hunt' as Mueller probe spills into second year

Of them are 20 White House officials, including lawyers and top presidential aides, and 28 people who worked with the campaign.

The team has also questioned at least six current and former top officials of the Department of Justice, including Attorney General Jeff Sessions and ex-FBI director James Comey; current and former US spy chiefs including CIA director Mike Pompeo; business figures tied to suspects, and Trump's friends and family members.

Key questions

Despite the Mueller team's silence, it is clear they are focused on three main issues.

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First - considering the multiple contacts with Russians by Trump campaign members - was anything promised or exchanged, and did anyone co-operate in Moscow's interference in the election? That theme also questions who was involved - and what Trump himself knew about those contacts.

Secondly, has the Trump family improperly melded their business ties with Russia, and other countries, with the work of the White House?

And third, did Trump attempt to obstruct the investigation when, among other actions, he fired FBI director James Comey, hinted at amnesty for Trump's national security advisor Michael Flynn and others, and attacked Mueller himself?

Should Trump be worried?

Donald Trump has been critical of the inquiry from the outset saying it "hurts our country terribly" and there's been continued speculation that he has wanted to fire Mueller.

On the one year anniversary of the probe, Trump tweeted that it was a "disgusting, illegal and unwarranted witch hunt".

Despite the disgusting, illegal and unwarranted Witch Hunt, we have had the most successful first 17 month Administration in U.S. history - by far! Sorry to the Fake News Media and “Haters,” but that’s the way it is! — Donald J. Trump (@realDonaldTrump) May 17, 2018

"There is no doubt that Team Trump, if not the man himself, was mindful of where such an investigation could lead," De Montfort University politics lecturer Clodagh Harrington writes.

In this piece for The Conversation, Ms Harrington says there is speculation that Trump may not be a "target" of the inquiry and unlikely to be indicted, but the inquiry has damaged his reputation.

A March poll by Pew Research found 61 percent were "very" or "somewhat" confident that Mueller's probe would be fair. Another poll a month later conducted by the Washington Post and ABC found 69 percent of Americans supported the investigation.

22 indictments

So far the investigation has issued 22 indictments. Sixteen were for Russian individuals and companies associated with online meddling in the 2016 election.

The other indictments indicate the Mueller team is aiming high: some campaign officials have been offered light charges in exchange for cooperating with the probe, suggesting they would be providing evidence against someone senior to them.

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Former Trump campaign chair Paul Manafort has notably been charged with multiple counts of money laundering, tax evasion and bank fraud, mostly related to work he did for Ukraine and Russian figures before joining Trump's election effort.

Manafort's longtime partner and deputy campaign chair Rick Gates was similarly charged - but then reached a plea deal on more limited charges, with a pledge to help Mueller.

READ MORE Trump blasts former aide Papadopoulos as a 'liar'

Michael Flynn admitted guilt to one count of lying to investigators on December 1 and also vowed to cooperate.

And campaign foreign policy advisor George Papadopoulos - who pleaded guilty about lying to the FBI about his campaign contacts with Russian officials - worked out a similar plea deal over one count of lying.

Meanwhile, partly on the Mueller team's recommendation, the FBI launched a separate investigation into Trump's longtime personal lawyer Michael Cohen, raiding his New York residences and offices.

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How much has it cost so far?

The Mueller inquiry is estimated to cost US taxpayers millions of dollars.

In its first four and half months, the probe burned through $US6.7 million, according to a US Department of Justice report released last year.

With such a broad scope and no end in sight to the investigation, the final cost is expected to be much higher.