Did members of Donald Trump's 2016 presidential election campaign collude with Russia to get him elected?
And did Trump, as president, engage in obstruction of justice?
Here is a look at the two questions:
Trump has denied -- repeatedly and heatedly -- that there was any collusion between members of his 2016 presidential election campaign and Russia.
"There's been absolutely no collusion," Trump said over the weekend of a probe he has denounced on multiple occasions as a "political witch hunt."
US intelligence agencies determined with "high confidence" in a report issued in January that Russia had sought to influence the 2016 race in Trump's favor.
But the report by the Central Intelligence Agency, Federal Bureau of Investigation, National Security Agency and Director of National Intelligence stopped short of stating that Trump's election campaign had colluded with Russian efforts to get him elected.
That was left to Mueller, a highly respected former director of the FBI, to resolve.
Mueller was named in May by acting attorney general Rod Rosenstein to look into whether there had been "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump."
So far, Mueller's team has charged four people but none of them has been accused of directly colluding with Moscow to get Trump into the White House.
Retired lieutenant general Michael Flynn, Trump's former National Security Adviser, pleaded guilty on Friday of lying to the FBI over his contacts with Russia's ambassador to Washington.
But those conversations occurred in December, after Trump was elected, and do not directly support the charge of collusion with Russia to swing the vote.
In exchange for a single charge and a potentially light sentence, Flynn did agree, however, to fully cooperate with Mueller's team.
This has led to speculation that Flynn may have more information to provide to investigators about any contacts between Trump's campaign and Russia.
"If there was coordination between the Trump campaign and the Russians I can't think of a person who would know more about that than Flynn," Republican Senator Lindsey Graham told CBS News.
"So it won't be long before we understand, one way or the other, whether Trump people colluded with Russia because I can't imagine it happening and Flynn not know about it," added the senator from South Carolina.
Also charged with lying to the FBI about his Russian contacts -- and also cooperating with Mueller -- is George Papadopoulos, a low-level foreign policy advisor to the Trump campaign.
Papadopoulos cultivated Russian contacts during the campaign and tried to arrange a Trump trip to Russia or a meeting with Russian President Vladimir Putin.
Trump's eldest son, Donald Trump Jr, and son-in-law Jared Kushner are also believed to be on Mueller's radar because of their contacts with Russians.
But they have not been charged with anything so far and nothing has emerged yet linking Trump himself directly to the Russians.
Obstruction Of Justice
Trump's firing of FBI director James Comey in May is at the heart of the accusations that the president may have engaged in obstruction of justice.
Comey had been conducting an FBI investigation into the allegations of Russian meddling in the election when he was abruptly dismissed by Trump.
The FBI director had also been looking into Flynn, who had been fired as national security adviser in February for lying to Vice President Mike Pence about his contacts with the Russian ambassador.
According to Comey, Trump urged him at a meeting in the Oval Office in February to halt an investigation into Flynn.
Such a request to an FBI director has been described as obstruction of justice by some legal scholars.
But Trump has denied making such a demand of Comey and engaged in a campaign to disparage the former FBI director since dismissing him in May.
"I never asked Comey to stop investigating Flynn," Trump tweeted on Sunday. "Just more Fake News covering another Comey lie!"
That tweet, however, followed one on Saturday which suggested Trump already knew Flynn had lied to the FBI when he asked Comey to drop the investigation.
"I had to fire General Flynn because he lied to the Vice President and the FBI," Trump tweeted. "He has pled guilty to those lies."
In an attempt to back away from the tweet, a Trump attorney, John Dowd, told the website Axios that he had drafted the tweet, not the president himself.
Dowd also reportedly claimed that a president cannot be guilty of obstruction of justice "because he is the chief law enforcement officer under (the Constitution) and has every right to express his view of any case."
That assertion may be on shaky legal ground.
As part of their impeachment proceedings, former presidents Bill Clinton and Richard Nixon were both accused of obstruction of justice and other offenses.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)