WASHINGTON — President Donald Trump is under federal investigation as part of Special Counsel Robert Mueller's probe, NBC News has confirmed.

At the center of the storm are accusations that the president may have engaged in “obstruction of justice,” a federal offense that involves intentionally trying to derail an investigation. But the situation is further complicated by the unique position held by the president, whose actions — according to one prominent legal theory — can only be prosecuted in office by Congress with impeachment.

Deputy Attorney General Rod Rosenstein appointed former FBI Director Robert Mueller as special counsel to oversee investigations into Russia's role in the 2016 election. Normally, the decision would have been up to Attorney General Jeff Sessions, but he recused himself from the matter shortly after acknowledging that he had failed to tell Congress during his confirmation process about a meeting with Russian Ambassador Sergey Kislyak.

Mueller is a widely respected figure in both parties whose role puts the probe further out of the White House's reach, but Trump and allies have already attacked his investigation, setting up the possibility the president might try to indirectly fire him if the investigation grows.

As lawmakers struggle to process the fast-moving story, let’s take a moment to review the basics of what the accusations around Trump mean and how things could proceed if more is found.

What is obstruction of justice?

There are a variety of federal crimes related to obstruction of justice, from threatening jury members to lying to investigators. In Trump’s case, reports and testimony surrounding his behavior toward former FBI Director James Comey could feed into a broad statute targeting anyone who intentionally “obstructs, influences, or impedes any official proceeding, or attempts to do so.”

The current allegations around Trump set off alarm bells when they first emerged because they could potentially be construed as Trump attempting to push Comey away from an investigation into his former National Security Adviser Michael Flynn.

What happens if it is found the president broke the law?

Charging the president with a crime is not like charging the average citizen with a crime, or even other top White House officials.

“Whether or not one can indict criminally a sitting president is an unanswered question,” Susan Bloch, a constitutional law professor at Georgetown, told NBC News. “It’s never happened and people are not sure whether it can.”

While it’s never been tested in court, many legal scholars, including Bloch, argue that the president is constitutionally protected from criminal prosecution as long as they occupy the White House. The prevailing opinion was expressed in a 1973 memo by President Richard Nixon’s Solicitor General, Robert Bork, who argued that then-Vice President Spiro Agnew could face charges stemming from his time as governor of Maryland, but that a president could only be charged after leaving office.

"When you put that all together with his statements and explanations and conduct, then common sense tells you that what he’s trying to do is shut down this investigation"

Instead, the job of prosecuting a president would likely be left to Congress. If investigations into Trump’s conduct turned up strong evidence of wrongdoing, the House could vote to impeach. The Senate would then try the president and, if two-thirds of members agreed, remove him from office.

But Congress is not a normal jury and the process could turn on political considerations as well as legal ones.

For one, the president’s party currently controls both the House and Senate, which sets the bar high for any move toward impeachment. But even if Democrats were to take over, there could be factors that cause them to hesitate as well.

Rep. Adam Schiff (D-CA), the ranking Democrat on the House Intelligence Committee, suggested in May that Congress would consider the national mood toward Trump along with any hypothetical charges.

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“Would the country, would the Congress, both Democrats and Republicans, view the president’s conduct so serious as to be disqualifying? Or would it be viewed as just a way to nullify the election by other means?” Schiff said on MSNBC’s "Morning Joe." “That's the ultimate test.”

Has anything like this happened before?

The last two presidents threatened with impeachment, Bill Clinton and Richard Nixon, both faced charges of obstruction of justice along with other accusations.

In Clinton’s case, he was found not guilty. Nixon resigned before the process could play out, but the House Judiciary Committee had already advanced articles of impeachment with bipartisan support. President Gerald Ford, who took over, subsequently pardoned Nixon.

It’s notable that in Nixon’s case, the “smoking gun” recording that preceded his resignation included audio in which the president appeared to order an aide to convince the FBI to drop its investigation of the Watergate break-in.

Michael Gerhardt, a law professor at the University of North Carolina who has written a book on the impeachment process, said there could be parallels between Nixon and Trump, but it was too early to know for sure. The charges against Nixon were far more extensive and included accusations of abuse of power.

“It doesn’t mean what Trump did is not a problem, but the facts are different,” Gerhardt said.

How does the latest news fit into this?

Let’s go over the most recent reports: According to Comey's testimony, the former FBI director wrote a memo documenting his meeting with Trump on February 14. The existence of the memo was first reported by The New York Times and confirmed NBC News shortly after Comey was fired.

The meeting came the day after Trump had fired Flynn for discussing sanctions with Kislyak, the Russian Ambassador, and misleading Vice President Mike Pence about the conversation.

According to Comey's testimony before the Senate Intelligence Committee in June, Trump spoke to him alone at the end of their meeting and said, "I hope you can see your way clear to letting this go, to letting Flynn go."

The White House has denied anything improper occurred, but Comey told Congress he interpreted Trump's statement as a message to "drop any investigation of Flynn in connection with false statements about his conversations with the Russian ambassador in December." He also said he leaked his account of the meeting through a friend in order to help build momentum for a special counsel.

In his own testimony, Sessions confirmed that he left the room at the end of Comey's meeting and that Comey expressed concern later. But Sessions indicated he did not interpret Trump's behavior, as relayed to him by Comey, as improper.

Alex Whiting, a Harvard law professor and former federal prosecutor, told NBC News in May that the initial reported account of the Comey meeting was a “game changer” because it could show that Trump not only acted inappropriately but also tried to block an ongoing probe. “It’s quite hard to find a proper purpose for that conversation,” Whiting said.

This is important, because proving criminal intent is a prerequisite for advancing obstruction charges and it can be a difficult hurdle.

Also potentially critical is that Comey's alleged version of events connect to earlier reports and actions suggesting that the president was disturbed by Comey’s investigations into Trump's associates and was willing to confront him over the issue.

The White House offered shifting explanations for Comey's removal, but Trump told NBC News’ Lester Holt that his disdain for the FBI’s ongoing investigation into his campaign’s possible ties to Russia, which Comey confirmed in March, helped inform his thinking.

In a meeting with Russian officials at the White House the day after Comey's firing, including Kislyak, Trump called Comey a "nut job" and that "he faced great pressure because of Russia" that had been "taken off," according to a New York Times report. The White House has not denied the remarks occurred.

Trump also called investigations into his campaign’s ties to Russia “a taxpayer funded charade” on Twitter the day before firing Comey. At the same time, Trump has said he wants the investigations to continue.

Shortly after Comey’s removal, sources close to Comey told news outlets, including NBC News, that Trump asked the FBI director to pledge his loyalty at a meeting in January. Comey confirmed this exchange in his Senate testimony, saying he tried to deflect the entreaty by offering his honesty instead.

After news of Comey's claims surfaced in May, Trump tweeted what appeared to be a threat in which he warned that Comey “better hope that there are no ‘tapes’ of our conversations before he starts leaking to the press!” More than a month later, White House has declined to confirm or deny whether Trump recorded their meetings, but has denied he asked Comey for his personal loyalty. Comey has testified he hopes tapes are available, which he believes would corroborate his account.

Okay, so how do we find out if the allegations are true?

There are a number of current and potentially future investigations that could help resolve the questions surrounding the president.

The FBI, under the watch of Mueller, is looking into the Trump campaign’s potential ties to Russia. Based on the latest reports, the investigation's scope now includes whether Trump himself violated the law. That story was first reported by the Washington Post.

Mueller's work should take place behind the scenes. But Congress is also looking into Russian interference as well as Comey's firing and it could produce more public information in the short term. Already the Senate has questioned Comey himself, Sessions and Rosenstein.

Leaders of the Senate Judiciary Committee have also asked the White House to provide “all White House records memorializing interactions with Mr. Comey relating to the FBI’s investigation of alleged ties between President Trump’s associates and Russia, or the Clinton email investigation, including all audio recordings, transcripts, notes, summaries, or memoranda.” That would presumably include the "tapes" Trump alluded to, if they exist.

But Trump could also potentially impact the investigation into whether he interfered with the investigation. While Sessions has recused himself from the matter and expressed confidence in Mueller, some allies of Trump have suggested he consider ordering Rosenstein to fire the special counsel.

Nixon tried something similar during Watergate. In the famous "Saturday Night Massacre," the attorney general and deputy attorney general both resigned rather than fire the prosecutor investigating the case. Eventually, Solicitor General Robert Bork followed through on the order, but the episode sparked a bipartisan backlash that led to a new independent prosecutor and ended up accelerating the president's downfall.

In an apparent reference to Mueller's probe, the president tweeted on June 15 that Americans were "witnessing the single greatest WITCH HUNT in American political history — led by some very bad and conflicted people!"

The president also has the power to unilaterally issue pardons, which could spark a political scandal if its seen as a coverup, but provide legal relief if he or close aides come into focus. President Gerald Ford pardoned Nixon for any potential crimes arising from Watergate, for example.