Paul Callan is a CNN legal analyst, a former New York homicide prosecutor and currently is of counsel at the New York law firm of Edelman & Edelman PC , focusing on wrongful conviction and civil rights cases. Follow him on Twitter @paulcallan . The opinions expressed in this commentary are his own

(CNN) Historians may one day remember May 9, 2017 as a fateful day in American history. In firing FBI director James Comey, the 45th President of the United States may well have set in motion a series of events that could lead to more controversy and potentially even his impeachment less than a year after being sworn into office.

President Trump and Attorney General Jeff Sessions set these events in motion by dispatching the highly respected former Acting Attorney General Rod Rosenstein to craft a rationale to fire Comey.

In a three-page memorandum , the best Rosenstein could do was to invoke what is now ancient political history -- Comey's alleged violation of Department of Justice policies in the handling of the Hillary Clinton email investigation. The alleged transgressions related to this investigation occurred months before Trump's Inauguration.

Of course, all of this was widely known and publicly discussed well before Trump made the decision to keep Comey in the position of FBI director.

The FBI director was obviously fired yesterday for something other than being too nice to Hillary Clinton by ignoring Department of Justice regulations.

Hints as to the real reason for the director's shocking termination can be found in a series of answers Comey gave when Senator Richard Blumenthal, D-Connecticut, interrogated him during a Senate Judiciary Committee hearing on May 3.

SEN. RICHARD BLUMENTHAL: Thank you, Director Comey, for being here, and thank you to you and the men and women who work with you at the FBI for their extraordinary service to our country, much of it unappreciated as you've wrote so powerfully in your opening statement. You have confirmed, I believe, that the FBI is investigating potential ties between Trump associates and the Russian interference in the 2016 campaign, correct?

COMEY: Yes.

BLUMENTHAL: And you have not, to my knowledge, ruled out anyone in the Trump campaign as potentially a target of that criminal investigation, correct?

COMEY: Well, I haven't said anything publicly about who we've opened investigations on, I briefed the chair and ranking on who those people are. And so I can't -- I can't go beyond that in this setting.

BLUMENTHAL: Have you ruled out anyone in the campaign that you can disclose?

COMEY: I don't feel comfortable answering that, Senator, because I think it puts me on a slope to talking about who we're investigating.

BLUMENTHAL: Have you -- have you ruled out the President of the United States?

COMEY: I don't -- I don't want people to over-interpret this answer, I'm not going to comment on anyone in particular, because that puts me down a slope of -- because if I say no to that then I have to answer succeeding questions. So what we've done is brief the chair and ranking on who the US persons are that we've opened investigations on. And that's -- that's as far as we're going to go, at this point.

BLUMENTHAL: But as a former prosecutor, you know that when there's an investigation into several potentially culpable individuals, the evidence from those individuals and the investigation can lead to others, correct?

COMEY: Correct. We're always open-minded about -- and we follow the evidence wherever it takes us.

BLUMENTHAL: So potentially, the President of the United States could be a target of your ongoing investigation into the Trump campaign's involvement with Russian interference in our election, correct?

COMEY: I just worry -- I don't want to answer that -- that -- that seems to be unfair speculation. We will follow the evidence, we'll try and find as much as we can and we'll follow the evidence wherever it leads.

This was obviously not the answer that the President expected to hear from his FBI director as he testified under oath before Congress and the American people. President Trump no doubt fully expected his director to say with utmost clarity and sincerity, "The President is not under investigation."

These words Comey refused to speak -- but it's clear from the bizarre language of Trump's letter terminating the director that the President wanted Comey to say them. "While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the Bureau."

Why would the President insert this phrase? He did so because the real reason Comey was being fired was his egregious violation of the Trumpian principle of absolute and unquestioning loyalty to President Trump.

By refusing to unequivocally deny even the possibility that Trump was being investigated, Comey demonstrated that he could not be trusted in the troubled times ahead.

Trump could have removed Comey from office at the start of his Presidential term, but he made a calculated judgement that Comey would owe the President for the substantial favor of allowing the director to remain in office.

After all, the director's public comments about the pending Hillary Clinton email investigation had clearly violated long-standing Department of Justice policy requiring no comment, particularly during election periods. They warranted Comey's dismissal even before Trump was sworn in. Comey was also a bit of a loose cannon, but he was generally thought to be scrupulously honest.

By keeping Comey on when he took office, what better card could Trump hold than a favor owed by the director of the FBI as investigators zeroed in on the Russian hacking of the presidential election of 2016.

All of this puts Trump in the first mile of the Richard Nixon road to impeachment. It took Nixon, a skilled and experienced hand at manipulating the levers of power, almost two full terms in office before his abuse of power was detected and effectively prosecuted.

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Trump's clumsy manipulation of the very same levers has resulted in his own version of the Saturday Night Massacre after less than five months in office.

Public opinion will compel the appointment of a special prosecutor and the talk of impeachment may soon follow. The sweetest sound that many Republicans could hope to hear would be Vice President Mike Pence taking the presidential oath.

It's always the cover-up that destroys those who abuse the power of high office and the trust of the American people. The nation is in for a rough ride in the months ahead.