Washington (AFP) - For two months, Robert Mueller -- the lanky, 72-year-old independent prosecutor investigating the Russia scandal -- has worked in virtual silence in a nondescript government office building in downtown Washington.

But even without saying a word, the former FBI director and no-nonsense prosecutor has deeply unnerved the occupants of the White House just eight blocks away, especially President Donald Trump, over where his probe is going.

Mueller has built a team of more than a dozen tough-as-nails investigators, including one expert in flipping mafia witnesses, a money laundering specialist who chased down a corrupt billionaire, and one of the country's most experienced Supreme Court litigators.

Since May, they have been quietly interviewing witnesses and collecting documents to establish whether there are links between top aides from Trump's campaign, members of his family, and possibly the president himself and Russian interference in the 2016 election.

After dismissing the probe for months as "ridiculous" and "fake news," Trump laid bare his concerns this week, lashing out at the Justice Department, from his hand-picked Attorney General Jeff Sessions on down the line, over the probe.

He took special aim at Mueller, making clear he intends to try to undercut and discredit the man who could bring down his presidency -- and possibly eventually remove him.

In an interview with The New York Times, Trump complained that one day after he interviewed Mueller to replace fired FBI chief James Comey, Mueller instead went and took the job of investigating the Russia scandal.

"The next day, he is appointed special counsel. I said, what the hell is this all about? Talk about conflicts?" Trump said.

"I have done nothing wrong. A special counsel should never have been appointed in this case."

- Unflappable -

Any prosecutor taking on the presidency has to shoulder an immense amount of political pressure, said Randall Samborn, an attorney who took part in a probe that targeted political heavyweight vice president Dick Cheney in the 2000s.

But if anyone should be able to handle that, Samborn said, it would be Mueller.

Mueller, a former Marine wounded in fighting in Vietnam, is also a veteran of tough prosecutions, including taking on former Panama president Manuel Noriega and mafia don John Gotti.

He took the helm of the FBI one week before the September 11, 2001 attacks. In the following years, he turned it into a potent counter-terrorism agency.

And in a now-legendary defense of rule of law, he and Comey faced down president George W. Bush in 2004 over a secret, illegal domestic surveillance program.

Risking being fired, they forced Bush to adjust his plans.

It's the kind of fortitude that has garnered Mueller praise from both Democrats and Republicans for years.

"I don't think there's a legitimate concern about Bob Mueller," said Ken Starr, whose 1990s investigation of Bill Clinton very nearly forced him from the White House.

"Mueller is a pillar of Washington's legal and political communities, which heavily overlap," said former prosecutor Andrew McCarthy in the conservative National Review.

- Sprawling investigation -

Since May, Mueller's team of hard-nosed and deeply experienced federal prosecutors, FBI investigators, spy-chasers and money-path followers have been talking to witnesses and amassing files, the only hints of their work coming from requests they send to their targets.

The investigation appears to have spread beyond the issue of collusion with Russia.

According to reports, Mueller is looking into Trump's past real estate business and his tax returns, possible money laundering by campaign aides, perjury and obstruction of justice, and other possible crimes.

The investigation - as well as parallel probes by the House and Senate Intelligence Committees -- have extended beyond campaign aides to Trump's inner circle, including son Donald Trump Jr. and son-in-law Jared Kushner.

Clearly off balance, the White House has recruited its own brigade of attorneys with expertise in constitutional law, criminal defense, and cold-blooded media counter-attack.

They have accused Mueller's team of being biased toward Trump's election opponent Hillary Clinton, and have assailed the breadth of the investigation.