Former FBI Deputy Director Andrew McCabe would be one of the top two or three key witnesses in any obstruction of justice case against President Donald Trump. On Friday, Trump’s Attorney General Jeff Sessions destroyed McCabe professionally in a way that could ruin his reputation for telling the truth.

These are the most important things to remember after Sessions fired McCabe for an undefined failure to be forthcoming during an investigation of his role in the bureau’s work on the Hillary Clinton email scandal and related inquiries.

The report documenting McCabe’s alleged misconduct was not made public. Instead, Sessions released a statement with vague assertions of wrongdoing. McCabe was subject to a Department of Justice inspector general’s report about his conduct, reportedly as it pertained to leaks to the Wall Street Journal about the FBI’s Clinton investigation at the height of the 2016 election. Michael Horowitz, the department’s IG who would have issued the report, is an Obama appointee with a sterling reputation.

Without the details of his report, though, it’s impossible to assess the validity of the firing and to what extent a public pressure campaign to attack one of the chief witnesses against Trump might have influenced the decision.

Again, that report was not released, but Sessions cited it as the reason for McCabe’s firing. The investigation into McCabe concluded, according to Sessions, that he “had made unauthorized disclosure to the news media and lacked candor—including under oath—on multiple occasions.”

There is no explanation about what lacking candor means, and Sessions does not go so far as to assert that McCabe lied under oath or at any time to internal investigators. Sessions also cited a recommendation from the FBI’s Office of Professional Responsibility in making the decision. Robin C. Ashton, an Obama-era hire, is the general counsel and head of that body. Sessions, in making the decision, offered that it was based on the “recommendation of the Department’s senior career official.”

McCabe denied that he sought to mislead investigators. “The idea that I was dishonest is just wrong,” he told the New York Times following the dismissal. Again, without knowing the actual misconduct, the most relevant fact here is one which McCabe highlighted.

“This is part of an effort to discredit me as a witness,” he told the Times.

“Here is the reality: I am being singled out and treated this way because of the role I played, the actions I took, and the events I witnessed in the aftermath of the firing of James Comey,” McCabe said in a public statement.

The firing of Comey, Trump’s alleged pressure on Comey to promise him “loyalty” and to drop an investigation of Trump’s former National Security Advisor Michael Flynn, and whether that might constitute obstruction of justice is reportedly one of the subjects of special counsel Robert Mueller’s investigation.

Comey has said under oath before Congress that Trump, in a private Oval Office meeting, tried to pressure him to drop the investigation of Flynn before Trump fired him. Trump has denied that, claiming that Comey lied in his sworn testimony. Trump himself, however, has vacillated on whether he would offer an interview to Mueller presenting his own version of events. Lying to investigators is a federal crime, as is lying to Congress. Trump also reportedly nearly fired Mueller in the days after he told reporters that he would be willing to tell Mueller, in essence, that Comey had committed perjury in an effort to frame him.

Trump knows that Comey’s version of events potentially places him in grave jeopardy for a finding that he obstructed justice. McCabe is reported to be one of the few men who can offer some corroboration as to Comey’s version of events. While he was Comey’s No. 2, McCabe was reportedly told by Comey about what happened in Trump’s meetings with the then-FBI director. In December, CNN reported that McCabe testified before the House Intelligence Committee that Comey had told him of those conversations—which were also memorialized in contemporaneous notes by Comey—shortly after they occurred.

The firing is not only a way to strip McCabe of his pension, but would be a perfect way to try to discredit McCabe in any future testimony against the president. Indeed, the president wasted no time in gloating over the firing on Friday and attacking the credibility of both Comey and McCabe.

“Andrew McCabe FIRED, a great day for the hard working men and women of the FBI - A great day for Democracy,” the president tweeted. “Sanctimonious James Comey was his boss and made McCabe look like a choirboy. He knew all about the lies and corruption going on at the highest levels of the FBI!” This is certainly what an attack on the rule of law would sound like.

“In the absence of the IG report, it’s impossible to evaluate the merits of this harsh treatment of a 21-year FBI professional,” the ranking minority member of the House Intelligence Committee, Adam Schiff, tweeted on Friday. “That it comes after the President urged the DOJ to deprive McCabe of his pension, and after his testimony, gives the action an odious taint.”

“The OIG’s focus on me and this report became part of an unprecedented effort by the Administration, driven by the president himself, to remove me from my position, destroy my reputation, and possibly strip me of a pension that I worked 21 years to earn,” McCabe wrote in his own statement.

McCabe had already pledged to retire, which he was set to do as of Sunday on a full pension. As McCabe and Schiff noted, the firing also comes after a public pressure campaign on Twitter by the president of the United States to attack McCabe personally, a campaign that represented a wish to ultimately have him fired. Whatever the merits of the dismissal—Fox News reports that a DOJ official says 19 FBI employees were fired for a lack candor in the last year—McCabe being stripped of that pension just as he was set to gain it sends an obvious signal to anyone in the DOJ who might stand up if and when Trump challenges the rule of law.

If they haven’t already, Mueller’s team should request that IG report first thing Monday morning to determine how this decision was made and if there was any motivation to further obstruct his investigation by anyone involved. He should also call as witnesses every single person involved in the investigation and the decision.

Earlier this week, the House Intelligence Committee cravenly shut down its own investigation of Trump, releasing findings so blatantly partisan that even some of the Republicans on the committee who supported the closing of the investigation rejected them. If the Senate Intelligence Committee wants to maintain that it is still running a credible investigation in its own inquiry, it too would investigate this potentially odious episode.