President Donald Trump has dismissed special counsel Robert Mueller's investigation as a "witch hunt" and a "hoax," and emphatically denied that he or anyone on his campaign colluded with Russia to interfere in the 2016 election.
But the news this week that Trump had considered firing Mueller last June has thrust another angle of the investigation into the spotlight: obstruction of justice.
Mueller's probe was tasked with investigating "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump," but the special counsel was also authorized "to prosecute federal crimes arising from the investigation" -- such as obstruction of justice.
The special counsel's interest in this lane of inquiry has come into focus amid a spate of developments this week -- in particular, Mueller's request to question Trump, his interest in the firing of FBI Director James Comey and the revelation that Mueller's team has already interviewed Attorney General Jeff Sessions.
Here's a look at the legal definition of the term "obstruction of justice," and its potential legal ramifications for Trump, which were initially raised following his decision to fire Comey in May.
What is the crime of obstruction of justice?
Obstruction of justice is a federal offense that covers any attempt by someone to corruptly "influence, obstruct, or impede" the "due administration of justice."
"The key question here is whether the President acted with corrupt intent and, to determine what his intent was, we have to look at all the facts and circumstances surrounding the case," explained former federal prosecutor Renato Mariotti, now a partner at Thompson Coburn.
Why would Mueller look into it?
Mueller's mandate includes obstruction because he has been ordered not only to investigate any potential links between Trump campaign associates and the Russian government.
It also includes "any matters that arose or may arise directly from the investigation," as well "any other matters" under the special counsel regulations, which include "perjury, obstruction of justice, destruction of evidence, and intimidation of witnesses."
Where's the evidence of obstruction?
Many have focused on Comey's claim that Trump asked him to drop the investigation into former national security adviser Michael Flynn, but some legal experts say that misses the larger point: the President fired Comey after he refused.
"I think the key is that the most significant step Trump appears to have taken is not his specific discussions with Comey, but his decision to fire Comey entirely because of his dissatisfaction with the shape and progress of the Russia investigation," explained Steve Vladeck, CNN legal analyst and professor of law at the University of Texas School of Law. "On Flynn, it devolves into a fight over what Trump intended and what Comey understood him to intend. Contrast that with firing Comey, where there's nowhere near the same mess about why he did it."
"Obstruction of justice is not a black-and-white offense, so reasonable minds may well disagree about whether asking Comey to go easy on Flynn rises to the level of what federal law prohibits," Vladeck added.
"But whether the President's conduct is or is not within the letter of the law is irrelevant; there's a really good reason why, for generations, presidents from across the ideological spectrum have respected the principle of not interfering in federal criminal investigations. That President Trump seems wholly indifferent to this principle, or the potentially devastating consequences of its demise, is the real scandal here."
Can a sitting president even be indicted?
Even if Mueller found evidence of obstruction, legal experts point to two opinions written by the Justice Department's Office of Legal Counsel (OLC) in 1973 and 2000, both concluding that indictment or criminal prosecution of a sitting President would impermissibly undermine the capacity of the executive branch to perform its constitutionally assigned functions.
Here's the conclusion from the 2000 memo written by then-assistant Attorney General Randolph D. Moss:
"Our view remains that a sitting President is constitutionally immune from indictment and criminal prosecution."
But others point out that the Supreme Court has never directly addressed the issue.
"The question of whether criminal indictment can precede impeachment has never been addressed by a court -- it's an open question," said William Yeomans, a 26-year veteran of the Justice Department and fellow in Law and Government at American University Law School.
"The OLC makes law for the executive branch," Yeomans added, but "their opinions are not binding anywhere else."
So what about impeachment?
Article II of the Constitution provides for removal of the President from office "on impeachment for, and conviction of, treason, bribery or other high crimes and misdemeanors."
Yeomans, in a recent op-ed for The Hill, wrote that obstruction is a well-established ground for impeachment and charges of obstruction of justice have been a "staple" of the last two substantial attempts to impeach a president.
And after the President fired Comey, Harvard Law School's Larry Tribe told Slate: "In my view, we have clearly passed that point, both as a technical matter" under law, "and, much more importantly, as a matter of what might be called the 'common law' of presidential impeachment, as established principally by the House impeachment and Senate trial of Bill Clinton and by the articles of impeachment of Richard Nixon."