One of the primary principles of criminal investigations is that, in their quest for truth and justice, investigators must remain independent and free of interference or influence. Such meddling can take many forms, from bribery and witness intimidation, to evidence tampering and outright lying, and are generally referred to as "obstruction of justice."
In the wake of President Donald Trump's firing of FBI Director James Comey last week, many are speculating that Comey's termination was an attempt to thwart investigations into Trump's campaigns and advisor's connections with Russia, and therefore amounts to obstruction of justice. But what federal statutes would cover Trump's actions, and what, specifically, do they prohibit?
Federal obstruction statutes are intentionally written to cover a wide range of behavior. For instance, 18 U.S.C. § 1505 prohibits attempts to "influence, obstruct, or impede the due and proper administration of the law under which any pending proceeding is being had before any department or agency of the United States, or the due and proper exercise of the power of inquiry under which any inquiry or investigation is being had by either House, or any committee of either House or any joint committee of the Congress." The language of the statute, as with most obstruction of justice laws, focuses more on the effect of an action, rather than list specific prohibited actions.
Comey's FBI was investigating communication between Trump's former National Security Advisor Michael Flynn and Russia, when the president reportedly asked Comey to close the investigation. Considering it came from the President of the United States and his boss, requesting an end to the investigation could, in and of itself, be seen as an attempt to influence Comey's "proper exercise of [his agency's] power of inquiry." In addition, Comey's subsequent firing makes it appear like Trump was trying to obstruct or impede that inquiry.
As an astute "former president" (and one who should know from obstruction of justice charges) pointed out, however, "Obstruction of justice isn't proving obstruction, it's proving corrupt intent." As it turns out, whoever is behind this quote from Richard Nixon's parody Twitter account is correct, in a sense. Under federal obstruction of justice laws, it's not enough to prove that a president, or anyone else, did something -- federal prosecutors must also prove that something was done "corruptly" or with the specific intent to obstruct an investigation.
For example, as the New York Times pointed out, normally permissible actions, like filing legal complaints, can be evidence of corrupt intent if they are aimed at government agents in the midst of an investigation.
In Trump's case, the president has the full legal authority to fire Comey, as he has dismissed other federal employees. Like nearly all other employees, Comey's employment is "at-will," meaning that he can be terminated for any reason or no reason at all (unless a contract states otherwise). That said, employees can't be fired for illegal reasons, and a firing with the intent to influence or end a criminal investigation would be illegal.
Demonstrating corrupt intent, however, can prove difficult. While several theories were floated to justify Comey's firing, Trump himself seemed to connect the dots in an interview with NBC's Lester Holt:
"And in fact when I decided to just do it, I said to myself, I said 'you know, this Russia thing with Trump and Russia is a made-up story, it's an excuse by the Democrats for having lost an election that they should have won'."
Whether that implication is enough to indicate a corrupt intention on the part of the president will probably be left up to federal prosecutors, if the accusations of obstruction of justice ever become more than that.