He is the behind-the-scenes investigator who has remained in the shadows for two years while his investigation has shaped the daily news agenda since 2017.

Today, he appeared out of nowhere — just like one of the 34 indictments he filed over the course of his investigation.

Just after 9:30am (local time), DC was buzzing with the unexpected news that Robert Mueller would be stepping up to the podium.

Two minutes before he spoke it was confirmed he would not be taking questions, which might have teased out some of the hints he dropped about US President Donald Trump and possible crimes.

In a voice that wavered a little at times, the methodical Mr Mueller spoke for just under 10 minutes.

The crux of it comes down to his comments about the potential obstruction of justice that made up half of his investigation.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 9 minutes 35 seconds 9 m Robert Mueller's media conference in full, saying it wasn't an option to charge Donald Trump

Obstruction 'of paramount importance'

It is an offence of which many have a poor understanding and firstly Mr Mueller set out why it is so important.

"The matters we investigated were of paramount importance," he said.

"It was critical for us to obtain full and accurate information from every person we questioned.

"When a person of investigation obstructs that investigation, or lies to investigators, it strikes at the core of the Government's efforts to find the truth and hold wrongdoers accountable."

Then the key line: Despite Mr Trump's claims of exoneration, Mr Mueller left live the possibility he was among the wrongdoers.

"If we had confidence that the President clearly did not commit a crime, we would have said so," he said.

But that said: "We did not however make a determination as to whether the President did commit a crime."

Then the acknowledgement the ultimate arbiter must be Congress.

"Under longstanding department policy a president cannot be charged with a federal crime while he is in office," he said.

"That is unconstitutional. Even if the charge is kept under seal and hidden from public view, that too is prohibited."

An opaque message from an opaque man

Just like the man himself who kept firmly out of the public eye while leading one of the most high-profile investigations of all time, the message is opaque.

Did he mean that further presidential indictment was warranted but prohibited?

If he doesn't have confidence to say the President didn't commit a crime, does that mean he believes Mr Trump did commit obstruction of justice?

Mr Mueller stopped and considered his words as he said: "It would be unfair to potentially accuse somebody of a crime when there can be no court resolution of the actual charge."

He did say that despite that, it was important to gather all of the information while memories were fresh.

Does that mean a charge could be laid after the President leaves office?

The soon-to-be-retired-to-private-life Mr Mueller did not answer questions and there was no doubting his word that he would not appear before reporters again.

Neither is he keen to appear before Congress, insisting that if called he would only speak to his published report and go no further than what is in it.

A problem for Pelosi

The one certainty is that he has made Speaker Nancy Pelosi's life much harder.

Democrats on the party's left have already been pushing for impeachment and they read Mr Mueller's statement as an invitation.

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Senator Cory Booker, a contender for the 2020 presidential election, called for impeachment for the first time.

Many in the party feel they should take advantage of their numbers in the House and begin impeachment proceedings.

Standing in their way is Ms Pelosi and political centrists.

Impeachment would of course fail in the Republican-controlled Senate and could backfire for Democrats, swinging sympathy behind Mr Trump ahead of the 2020 election.

Ms Pelosi remembers the public sentiment shifting Bill Clinton's way when he was impeached for perjury for lying about his famous affair.

It is feared impeachment would energise Mr Trump's core base.

It could also see him head to 2020 painting his opponents as more concerned with inside-the-beltway politics rather than the rapidly improving US economy.

Mr Mueller obviously chose his words carefully, but his intention and opinion remained as unclear as ever.

And it appears this is as much as we will ever hear from the man himself.

Mr Trump and the White House were unusually understated, with the President even ending his response tweet with a thank you.

His intentions and meaning are rarely opaque and for him, today's farewell press conference is "case closed".