What to watch for in the Mueller probe

"Congratulations America, we are now into the second year of the greatest Witch Hunt in American History...and there is still No Collusion and No Obstruction," President Donald Trump tweeted Thursday.

Trump's supporters argue that special counsel Robert Mueller's failure so far to publicly report a conspiracy by campaign aides to cooperate with Russia means there was no collusion.

Many dismiss the other major thrust of the investigation -- into whether Trump obstructed justice -- by arguing that a President can lawfully fire an FBI director at any time.

But the White House spin skips over crucial unanswered questions that will dictate the outcome of the investigation, and possibly even the fate of the Trump presidency itself.

Mueller appears to be building a classic outside-in criminal investigation, which means that the most dramatic developments are ahead and the most senior members of Trump's inner circle would be unwise to breathe too easily just yet.

What is next?

Many legal experts believe that Mueller will soon unveil indictments against hackers accused of burrowing into email accounts of the Democratic National Committee and the Hillary Clinton campaign that were then published by WikiLeaks.

Indictments could be largely symbolic since there is little chance anyone targeted in Russia linked to Moscow's intelligence services will ever stand trial in the United States.

But the real question is whether anyone in the US -- inside or outside the Trump campaign -- is also indicted in the hacking scheme.

"Is it just a story of Russia and WikiLeaks conspiring together to engage in computer hacking and then releasing information or was someone in Trump's orbit involved in that?" said Jens David Ohlin, vice dean of Cornell Law School.

Former Trump campaign adviser Roger Stone has told CNN that his actions were being examined by Mueller's team. On Wednesday, it emerged that Mueller had subpoenaed Stone's former social media adviser Jason Sullivan. The Wall Street Journal has reported the special counsel is looking into potential ties between Stone, WikiLeaks and its founder Julian Assange.

A hacking indictment could be an appetizer for bigger questions ahead -- including a potential showdown over whether Trump will agree to an interview with Mueller.

The President has repeatedly said he wants to talk, though his lawyers worry about his propensity to be loose with the facts.

If he refuses, Mueller would have to decide whether to subpoena the President and then litigate, potentially all the way up to the Supreme Court, to compel his testimony if necessary.

Conventional wisdom is that Mueller will not wrap up before talking to Trump. But given that the President could plead the Fifth, Mueller could judge that prolonging the investigation for many months with a legal fight could ultimately be futile.

A year into the probe it's still not clear how Mueller will come down on two central questions: Was there a conspiracy involving Trump campaign aides and Russia to interfere in the election? And did the President obstruct justice, including in the firing of former FBI Director James Comey?

Some experts believe that Mueller has all but completed the obstruction piece of his investigation and could report to Deputy Attorney General Rod Rosenstein soon.

Rudy Giuliani is arguing that even if there were cooperation between Trump aides and Russia on getting dirt on Democrat Hillary Clinton, it would not be unlawful.

"There is nothing illegal about that. Even if it comes from a Russian or a German or American, it doesn't matter," Giuliani said Wednesday on the "Ingraham Angle" on Fox News.

To show Trump obstructed justice, Mueller must probe the President's motivation in firing Comey and prove he acted with corrupt intent. Many analysts believe there is already sufficient evidence for a strong obstruction case, including in the President's own statements and Comey's memos detailing his meetings with the President. It is not known if there is any evidence that is not publicly available.

The unknown unknowns

It is impossible for anyone outside Mueller's watertight investigation to know where the special counsel is headed.

That's why everyone has been astonished when he has unveiled indictments replete with detail and texture that suggest that whatever his conclusions, he will miss nothing.

It is unknown, for instance, whether Mueller is pursuing a case against key players around Trump -- including his son Donald Jr. or his son-in-law, Jared Kushner. Mueller's team has interviewed Kushner -- but only on his dealings with former national security adviser Michael Flynn.

Another big unanswered question is how Mueller will use the testimony of former Trump aides who have agreed to plea deals, including Flynn, former deputy campaign manager Rick Gates or campaign foreign policy adviser George Papadopoulos.

Logic suggests that Mueller knows the trio has evidence that could be used to incriminate people higher up the Trump campaign chain of command in future cases.

Then there is the question of whether Mueller is actively probing Trump's private finances and those of his family business to see whether there is any compromising material that could have been used in any Russian effort to influence him.

"I don't think any of us has the full picture. The question is, is there more there or not? I think that part of the investigation is not even close to being wrapped up," said Ohlin.

CNN has reported that Mueller's investigators have asked witnesses at the grand jury about Trump's business activities in Russia during the 2016 campaign.

It emerged in March that Mueller had subpoenaed Trump Organization documents, raising questions over whether the special counsel was getting close to the red line that Trump decreed over the potential probing of his family finances by the special counsel.

In many ways, the Mueller investigation resembles an iceberg, with much of its mass concealed.

His investigation is certain to get drawn into the political tumult around the midterm elections. Given how Comey's investigation into Hillary Clinton got sucked into the 2016 election, Mueller may feel pressure to avoid major moves in the run-up to November.

That could mean any new indictments could come in the coming months.

Few analysts believe the entire investigation could be finished by Election Day. But if it continues, conservatives are likely to accuse the special counsel of hoping for a Democratic-led House, which would be more open to impeachment proceedings than the current GOP majority.