WASHINGTON — Nearly two years into his investigation, special counsel Robert Mueller has not accused any member of the Trump campaign of conspiring with the 2016 election interference effort — and it's not clear whether he will.

But legal experts, along with the congressman leading the House Russia investigation, tell NBC News that the most important question investigators must answer is one that may never have been suitable for the criminal courts: Whether President Trump or anyone around him is under the influence of a foreign government.

"It's more important to know what Trump is NOW than to know what he did in 2016," said Martin Lederman, professor at the Georgetown University Law Center and former deputy assistant attorney general in the Department of Justice's Office of Legal Counsel during the Obama administration. "It's more important to know whether he has been compromised as president than whether his conduct during the campaign constituted a crime."

Rep. Adam Schiff, D-Calif., then ranking member of the House Permanent Select Committee on Intelligence, leaves a secure area where the panel meets at the Capitol, on Feb. 5, 2018 J. Scott Applewhite / AP

Whether Mueller will answer that question in the absence of criminal charges is unclear. But in an interview with NBC News, House Intelligence Committee Chairman Adam Schiff said he is steering his investigation in a new direction to focus on it — and he will demand any relevant evidence compiled by the FBI or Mueller's team.

The California Democrat also expressed concern that Mueller hasn't fully investigated Trump's possible financial history with Russia.

"From what we can see either publicly or otherwise, it's very much an open question whether this is something the special counsel has looked at," Schiff told NBC News.

Schiff said the public testimony from former Trump lawyer Michael Cohen that in 2016 Trump stood to earn hundreds of millions of dollars from a secret Moscow real estate project is a staggering conflict of interest that must be fully explored.

"I certainly agree that the counterintelligence investigation may be more important than the criminal investigation because it goes to a present threat to our national security — whether the president and anybody around him are compromised by a foreign power," Schiff said. "That's not necessarily an issue that can be covered in indictments."

In fact, most FBI counterintelligence investigations don't result in criminal charges, experts say, because they tend to involve secret intelligence that either can't be used in court or doesn't add up to proof beyond a reasonable doubt. If the FBI assesses that a government official is compromised by a foreign adversary, officials often will quietly remove that person from a sensitive role or wall him or her off from classified information.

Obviously, none of that is an option for the president of the United States.

No official action was taken after Trump was accused of giving highly classified information to the Russian foreign minister in the Oval Office in 2017. As the president, he has the legal right to spill secrets to whomever he wants.

The White House has long insisted that the notion of a president in thrall to the Kremlin is ridiculous, pointing to the sanctions the Trump administration has levied on Russia in response to cyber attacks, election interference, and its actions in Ukraine.

Trump defenders complain that those who are now focusing on foreign influence have "moved the goalposts" — shifting emphasis to the issue of foreign compromise now that criminal charges involving "Russian collusion" seem less likely.

Trump has criticized Schiff’s approach, saying in a Feb. 7 tweet, "So now Congressman Adam Schiff announces, after having found zero Russian Collusion, that he is going to be looking at every aspect of my life, both financial and personal, even though there is no reason to be doing so. Never happened before! Unlimited Presidential Harassment."

But the question of Trump's motives regarding Russia has always been front and center for the FBI, as former Acting Director Andrew McCabe made clear in a recent round of media appearances. Neither Trump nor any of his supporters has been able to quell questions about the president's embrace of Russian President Vladimir Putin, including Trump's seeming unwillingness to criticize the Russian autocrat.

Robert Mueller, then director of the FBI, testifies on Capitol Hill in 2011. Brendan Hoffman / Getty Images file

McCabe, who was fired for lack of candor in an unrelated matter, alleged that the president disputed intelligence that a North Korean missile could hit the United States, saying, "I don't care. I believe Putin."

That allegedly happened behind closed doors, but few will forget the public spectacle of Trump siding with Putin over his intelligence community on the question of U.S. election interference at last year's Helsinki summit, telling the world: "President Putin says it's not Russia. I don't see any reason why it would be."

McCabe said he could not rule out that the president was, in essence, a Russian asset. Trump has called McCabe a liar and "a disgrace to the FBI."

"What Americans should be concerned about is whether the president's Russia policy is not dictated by our national interest but is dictated by his desire to make hundreds of millions of dollars off a tower in Moscow," Schiff said.

In the beginning, a counterspy probe

The FBI's Russia investigation began in July 2016 as a counterintelligence investigation into Trump campaign aides. Current and former officials say the case involved agents whose expertise is counterspy cases and agents who worked criminal matters.

After Mueller took over the investigation and began filing criminal charges, the public and the news media came to view the probe largely through the lens of the criminal justice system. But the counterintelligence dimension never diminished.

So now Congressman Adam Schiff announces, after having found zero Russian Collusion, that he is going to be looking at every aspect of my life, both financial and personal, even though there is no reason to be doing so. Never happened before! Unlimited Presidential Harassment.... — Donald J. Trump (@realDonaldTrump) February 7, 2019

The FBI was looking for crimes, but was also using intelligence tools to ferret out and thwart Russia's efforts to interfere in American democracy. And it was collecting intelligence that the government would likely never want to use in court, current and former officials tell NBC News.

In May 2017, Trump fired FBI Director James Comey, told NBC News anchor Lester Holt he did so with Russia on his mind, and told the Russian foreign minister the firing had relieved "great pressure" on him. That led the FBI to open a counterintelligence investigation into the president specifically, according to McCabe. It was married to a criminal investigation examining possible conspiracy with Russian election interference and possible obstruction of justice, McCabe said.

After Mueller in July 2018 accused Russian intelligence officers who hacked the Democrats of conspiring against the United States, many expected that he eventually might charge members of the Trump campaign with participating in that conspiracy.

That hasn't happened. To the contrary, some of the key figures seen as likely participants in any Russia conspiracy — Mike Flynn, Paul Manafort, Rick Gates, Michael Cohen — have been charged with other offenses in cases that made no allegation of any "collusion" with Russia. Two of them, Manafort and Cohen, have been sentenced.

But just because they haven't been charged criminally doesn't mean there isn't evidence they were influenced by Russia, said Frank Figliuzzi, the former top counterintelligence official at the FBI and an NBC News contributor.

"I cringe when I see people trying to apply criminal metrics to a counterintelligence case," he said.

More often than not in counterintelligence cases, Figliuzzi said, the FBI makes a determination that a person has been subject to foreign influence without pursuing criminal charges. For example, he said, if the bureau finds that a top general has developed an inappropriate relationship with a Chinese intelligence asset, that general likely wouldn't be charged with espionage or other crimes. But he almost certainly would be fired.

In Trump's case, longtime students of his real estate career have raised questions about how he generated large amounts of cash in recent years to make big purchases of golf courses and other assets, and of why the German bank Deutsche Bank — which has been fined for laundering Russian money — made loans to him when, by all accounts, no other bank would. There is no public evidence linking Trump’s relationship to Deutsche Bank with the allegations of laundering money for wealthy Russians.

According to the Washington Post, the Justice Department's Public Integrity Section is investigating how Saudi Arabia and other countries sought to influence the Trump Administration through relationships with a key fundraiser, Elliott Broidy. Through a spokesman, Broidy has not denied working with those countries, but he denies any wrongdoing.

It has long been presumed that Mueller would investigate these questions, to determine the extent of Trump's financial history with Russia or other foreign entities that now have a stake in his policy decisions. Trump is the only president in recent history not to have released his tax returns, and the only one to retain a stake in a business empire.

How far did Mueller go?

But Schiff told NBC News he is not convinced that Mueller's Russia investigation — tasked with examining whether there was any coordination between the Trump campaign and Russian election interference — delved deeply into Trump's personal finances.

"The concern I've had in terms of the scope of the Mueller investigation is that the president has tried to draw a red line around certain aspects of his finances," Schiff said. "That's not a line that can be observed and still protect the country."

The special counsel’s office declined to comment.

Schiff noted that the New York Times reported that Trump demanded that Mueller be fired in December 2017 after news reports suggested the special counsel had sent a subpoena to Deutsche Bank. After Trump's lawyers were assured by Mueller's office that the reports were not accurate, the president backed down, according to the Times. The president has called reports he sought to fire Mueller "fake news."

But if Mueller hasn't looked at Trump's relationship the German bank, Schiff said, "they have not done a diligent investigation of money laundering."

He added: "If the president has been successful in chilling the DOJ from looking at his finances, then the Congress needs to do it… Any way in which this president or those around him might be compromised by a foreign hostile power is front and center in our probe."

David Kris, a former Justice Department national security lawyer and founder of Culper Partners consulting firm, worked closely with Mueller when he was FBI director, and knows him well. He told NBC News he believes that Mueller would have taken whatever steps necessary to examine the question of whether Trump had an improper financial relationship with Russia or Russian oligarchs.

"I would trust Mueller to be competently thorough," he said.

It's possible, however, that Mueller didn't believe his mandate allowed him to rummage through Trump's financial history, he added.

Either way, Kris said, Schiff and Congress are on solid legal footing in expecting that Mueller and the FBI will brief them on the results of the counterintelligence investigation into the president — regardless of what criminal charges Mueller files or what he puts in his report.

Leaving aside the special counsel regulations, the executive branch is required by law to keep the House and Senate intelligence committees "fully and currently informed" on important intelligence matters, Kris noted. There is also a requirement that the Justice Department tell Congress about any instances in which a prosecution was halted over concerns about intelligence sources and methods.

Figliuzzi said he frequently briefed Congress on major counterintelligence cases as FBI counterspy chief. He said it's essential that Mueller tell the Congress what, if anything, he has found pointing to possible Russian or other foreign influence on anyone in the Trump administration.

"This is an independent reporting obligation that the intelligence community has," Schiff said. "I do think that the special counsel regulations certainly permit the sharing of this information with the Congress and we're going to insist on it."

Schiff said he is particularly concerned about the Trump Tower Moscow project, the real estate development Cohen was pitching to the Kremlin while Trump was running for president.

He noted that when it first emerged that Cohen had emailed Putin's office seeking help, Putin's spokesman, Dmitri Pescov, said he never answered the email. But it later emerged in court documents that an assistant to Pescov did respond, emailing Cohen and asking him to call, which he did.

"So here we had the Kremlin facilitating a cover up by the president of the United States," Schiff said. "This needs to be exposed."