At this point the facts are well known: Tuesday night, President Trump fired the FBI director, who was leading a criminal investigation into the Trump campaign’s possible coordination with the Russians.

Trump did so days after then-FBI Director James Comey reportedly asked for more money to support the investigation. And Trump did so, in part, based on a memo written by the newly appointed Deputy Attorney General, Rod Rosenstein, who has been tasked with overseeing the Russian investigation on behalf of the Justice Department.  

The case for a special prosecutor could not be more clear.  

{mosads}Trump and his spokespeople claim that the investigation will continue unhindered. But if anyone really thinks that the prosecutors and investigators working on this case aren’t influenced by Trump’s actions, they are living in La La Land. If the head of the FBI could be fired for crossing the president, what could happen to the rank and file?  

And while Republicans and Democrats alike (myself included) had high hopes that Mr. Rosenstein could engage in the day-to-day oversight of this investigation with the kind of impartiality and credibility that is so sorely needed, those hopes are now dashed. At best, Rosenstein demonstrates a total lack of judgment in failing to emphasize to the president both the political and legal risks (including exposure to possible obstruction of justice charges) that would stem from the firing of the chief investigator in the pending criminal investigation.  

Trump: Accusations of collusion between my campaign and Russia “set up by Democrats” https://t.co/O0O31vfUvM pic.twitter.com/ZK1iWjO9En

— The Hill (@thehill) May 12, 2017

Worse, Rosenstein may have made the case at the President’s direction despite his own personal misgivings — thereby explaining his memo’s extensive reliance on others’ views about Comey and obliterating any claim of independence.  

This takes me to the other key reason why a special prosecutor is needed — to assess whether the president (and perhaps others advising him) engaged in the obstruction of justice. To use the words of the statute, did Trump (and perhaps others advising him) “corruptly … endeavor() to influence, obstruct, or impede the due and proper administration of the law” with respect to the pending investigating?

If firing the chief investigator in a criminal probe that leads to one’s inner circle doesn’t count as a form of influence, I don’t know what does. The question then becomes did Trump do so with “improper purpose?”  

Did he do so because he was suddenly concerned about the way Comey handled the Hillary Clinton email investigation? (Almost certainly not.) Did Trump do so because he decided Comey “wasn’t doing a good job” anymore, as he claimed on Wednesday, even though Press Secretary Sean Spicer insisted that the president had full confidence in Comey just one week earlier? (I’m still not convinced.) Or did he do so with the aim of tamping down — i.e., improperly influencing — the investigation? (Possibly.)

House Judiciary Dems call for hearings on Trump’s firing of Comey https://t.co/dKvuvznIv4 pic.twitter.com/Rk2kwa7C97

— The Hill (@thehill) May 12, 2017

Obstruction of justice is an incredibly serious charge. Even suggesting that it should be considered is a very serious matter. But so is firing the FBI director in the midst of a criminal probe that potentially implicates one’s own presidential campaign. (And just in case anyone was fooled, Trump’s repeated insistence that he had assurances he wasn’t personally being investigated doesn’t help him in this matter. Obstruction of justice charges don’t depend on whether or not he is the direct target of the investigation; after all, there are obvious reasons why one might want to tamp down the criminal probes of close friends and associates as well.)

Deciding whether such charges are in fact warranted will require a careful, thorough, and independent investigation and evaluation of the facts — examining, among other things, the communications between the president, the attorney general, and deputy attorney general that led up to Comey’s firing. This is something that only someone that is freed from the day-to-day supervision of these same individuals can be expected to do.  

“Is Trump guilty of obstruction of justice? That depends…” https://t.co/jAhvYO0Vt1 pic.twitter.com/iX4fRvEdau

— The Hill (@thehill) May 11, 2017

Appointment of a strong and independent Comey replacement also will obviously be key. But the FBI director, no matter how forceful, is not the one that ultimately decides whether to bring charges. The prosecutor does.   

In fact, the best, and perhaps only, way for the Department of Justice to regain credibility in the matter is to appoint a tough and credible special prosecutor. And then stay out of his or her way. Congress should insist on it.

Of course, the appointment of a special prosecutor doesn’t guarantee we get the full, thorough, and independent investigation needed. A special prosecutor is still, ultimately, under the control of the Justice Department officials that make the appointment. Even special prosecutors can be fired. And there is no guarantee that any of the findings will ever see the light of day (which is why a special commission also is needed). But at this point, it is only credible prosecutorial option available.

Unless, of course, impeachment charges are filed …

Jennifer Daskal is an associate professor at American University Washington College of Law and was formerly counsel to the Assistant Attorney General for National Security at the Department of Justice.  

The views expressed by contributors are their own and are not the views of The Hill.