Donald Trump
U.S. President Donald Trump leaves the MedStar Hospital Center after a visit to injured victims, after a gunman opened fire on a baseball field where Republican lawmakers were practicing for a charity game, in Washington, DC, U.S. June 14, 2017. Carlos Barria/REUTERS

Special Counsel Robert Mueller is investigating President Donald for obstruction of justice, according to the Washington Post. The Wednesday story had the president very upset on Twitter the folllowing day.

“They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice,” Trump tweeted.

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Mueller is a former FBI Director and was appointed by Deputy Attorney General Rod Rosenstein to investigate alleged connections between the Trump campaign and Russia. Attorney General Jeff Sessions recused himself from the investigation because he was involved in the campaign. The specter of obstruction of justice began when Trump abruptly fired FBI Director James Comey. But what is obstruction of justice?

The federal law penalizes getting in the way of federal investigations. One federal statute states it is a crime for “whoever corruptly obstructs, influences, or impedes any official proceeding, or attempts to do so.” There are several laws that cover obstruction, and threatening communication is included in what is considered impeding an investigation. Intimidation is included as well. The laws are broad, allowing for interpretation.

Congress is responsible for trying the President in cases of obstruction.

Comey appeared before the Senate Intelligence Committee last week to answer questions about his firing. The former FBI director claims he was fired because of the Russia investigation. In an Interview with Lester Holt on NBC, Trump said he was thinking about Russia when he fired Comey.

Comey also testified that in a private setting Trump said to him, “I hope you can see your way clear to letting this go, to letting (Michael) Flynn go. He is good guy. I hope you can let this go.”

Comey was investigating former national security adviser Flynn as part of a probe into Russian involvement into the Presidential election last year, and if there was any possible collusion between elements of the Trump campaign and Russia.

Trump is not the first president to be investigated for obstructing justice. The most famous case was former Republican President Richard Nixon. He was investigated for a cover-up of a break-in at the Watergate Hotel in Washington D.C. The Democratic party had a headquarters at the hotel and the break-in was by members of the Committee to Re-elect President Nixon. It was discovered that Nixon knew about the break-in and attempted to impede the subsequent investigation.

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The attorney general at the time, Elliot Richardson, appointed a special prosecutor, Archibald Cox, to investigate Nixon. Nixon then commanded Richardson to fire Cox, Richardson refused and resigned in protest. Deputy Attorney General William Ruckelshaus was asked to fire Cox, but he also refused and resigned in protest. The resignations and eventual firing of Cox are known as the Saturday Night Massacre.

Former President Bill Clinton was also investigated for obstruction stemming from an affair with White House intern Monica Lewinsky, his testimony about the affair during a sexual harassment against him.