Shortly after the appointment of special counsel Robert Mueller in May, acting FBI Director Andrew McCabe told several of the highest-ranking managers of the bureau they should consider themselves possible witnesses in any investigation into whether President Donald Trump engaged in obstruction of justice, according to two senior federal law enforcement officials.

McCabe has told colleagues that he too is a potential witness in the probe of whether Trump broke the law in trying to thwart the FBI’s investigation into whether the Trump campaign colluded with the Russian government to defeat Hillary Clinton in the 2016 election.

Two senior federal law enforcement officials have told me that the new revelations illustrate why they believe the potential case against Trump is stronger than outsiders have thought.

“What you are going to have is the potential for a powerful obstruction case,” a senior law enforcement official said. “You are going to have the [former] FBI director testify, and then the acting director, the chief of staff to the FBI director, the FBI’s general counsel, and then others, one right after another. This has never been the word of Trump against what [James Comey] has had to say. This is more like the Federal Bureau of Investigation versus Donald Trump.”

Trump and his supporters have long argued that it would be difficult, if not impossible, for the special counsel to bring an obstruction case against Trump. The case would rely on the word of one man versus another, that of the president of the United States versus the FBI director he fired. But this was never the case.

Including Comey, as many as 10, and possibly more, of the nation’s most senior law enforcement officials are likely to be questioned as part of the investigation into whether Trump committed obstruction of justice, according to two government investigators with firsthand knowledge of the matter. Comey’s notes on his conversations could also be used as evidence, according to many reports.

The White House declined to comment. First contacted by email by on July 27, White House spokesperson Kelly Love responded late Wednesday saying, "This would be a question for outside counsel." Love did not name which of the president's many lawyers to contact. Marc E. Kasowitz, an attorney for the president, did not respond to a phone message Wednesday evening. The FBI also declined to comment.

FBI agents are experienced witnesses who routinely testify in high-pressure cases. Plus, the FBI itself is a rare public institution that is widely respected and trusted by the American public. The witness list and breadth of possible evidence, including notes Comey and several other senior FBI officials made at the time, could add up to a much stronger obstruction of justice case than Trump ever could have imagined.

Among those who McCabe and other law enforcement officials have privately believed are potential witnesses are six of the highest-ranking officials of the agency: They include McCabe himself; Jim Rybicki, Comey’s chief of staff; James Baker, the general counsel of the FBI; David Bowdich, who as the FBI’s associate director is the agency’s third-highest official; and Carl Ghattas, the head of the FBI’s national security division; and a legal adviser to McCabe. McCabe was deputy director of the FBI until May, when he became acting director after President Trump fired Comey.

Attorney General Jeff Sessions, Deputy Attorney General Rod Rosenstein, and a third senior Justice Department official are believed by law enforcement officials to be crucial fact witnesses in the obstruction probe. Their testimony is likely to support Comey and harm Trump, according to investigators and outside experts.

Mueller's case is looking stronger than Trump surrogates say

In May, Mueller was appointed special counsel to investigate whether Trump colluded with the Russian government to help defeat Hillary Clinton during the 2016 presidential election. A related area of inquiry for the special counsel is whether Trump obstructed justice when he allegedly asked Comey to shut down his inquiry of Trump’s former National Security Adviser Michael Flynn.

Trump made sure he and Comey were alone when he allegedly pressured the then-FBI director to curtail the FBI’s Russia investigation. At a private White House dinner on January 27, Trump allegedly pressed Comey to pledge his personal loyalty. The dinner came right after the president learned Flynn was under criminal investigation.

Later, on February 14, Trump allegedly leaned on Comey privately in an Oval Office meeting to shut down the FBI’s investigation of Flynn. Comey did not drop the investigation or take other steps Trump requested that the then-director of the FBI felt were improper. Trump then fired Comey on May 9.

Mueller is investigating whether Trump’s pressure on Comey to shut down his investigation — combined with other efforts to thwart the investigation, including firing Comey — are an obstruction of justice. As such, Comey is the central witness against Trump in any such obstruction investigation. That Trump was ordinarily alone with Comey when these various incidents occurred has led Trump and his surrogates to argue that it would be difficult for any obstruction of justice case to be brought because it would be based solely on Comey’s word.

“We have to keep in mind that is one person’s record of what happened,” Republican National Committee Chair Ronna Romney McDaniel said on Fox News in one typical comment repeated by White House surrogates. “The only two people who know what happened in these meeting are the president and James Comey.”

But even though Trump took great pains to try to be alone with Comey when they spoke, Comey regularly spoke to the six high-ranking FBI managers, often right after a distressing conversation with Trump about the Russia probe.

Comey spoke to these FBI officials almost always within 24 to 48 hours after such a contact took place, according to two senior federal law enforcement officials. A person familiar with the matter told me they know for certain there were at least eight such conversations — and likely more than a dozen — that Comey had with these high-ranking FBI managers, sometimes one on one, sometimes in groups of several officials. More than one such meeting was longer than an hour.

And in at least one previously unreported instance — that of a phone conversation between the president and Comey, during which Trump pressed Comey to say that Trump wasn’t personally under investigation — Rybicki, Comey’s chief of staff, was present for the entirety of the phone call.

Trump had unexpectedly called Comey while Comey was in a meeting with Rybicki. As Trump and the then-FBI director spoke, Rybicki stayed put and listened to the entirety of Comey’s side of the conversation, according to Comey’s testimony to Congress and a senior federal law enforcement official.

In addition, Comey often emailed Rybicki accounts of his troublesome discussions with Trump about the Russia investigation — if not immediately after, sometimes the same day, according to a senior federal law enforcement official.

Baker, the FBI general counsel, took methodical notes during his discussions with Comey and others in the FBI hierarchy about Trump’s efforts to thwart the FBI’s investigation, according to these same sources.

Law enforcement officials are likely to be questioned

I interviewed current and former law enforcement officials, including some who, though not directly involved in the investigation, have held key positions working for independent counsels or special prosecutors investigating earlier presidents. They told me they agree with McCabe’s assertions that the senior FBI managers are almost certainly to be questioned for any investigation of President Trump for obstruction of justice.

Sam Buell, a Duke University law professor who has previously served as a federal prosecutor in New York, Boston, Washington, DC, and Houston, similarly told me that Mueller will almost certainly interview all six senior FBI officials that Comey confided in, as well as Sessions and Rosenstein: “In any high-stakes matter, you are going to want to talk to anyone in the vicinity of a conversation. It doesn’t mean that they end up as trial witness. But at an investigative stage, you are going to talk to all of these people. You want their stories locked in. You want to know if what they have to say would help you or hurt you.”

John Keker, who during the Reagan and George H.W. Bush administrations prosecuted retired Lt. Oliver North for the Iran-Contra special prosecutor, explained to me: “Think of any crime. The defense might make the case that the accuser made it up. The questions for the witness are: ‘Did you just make this up?’ ‘Are you just saying this now?’ ‘Why didn’t you say something before?’ ‘Whom did you say something to? Did you write it down?’

“But if they told people when it happens, it makes their story more plausible. It helps their credibility. In this case, the people Comey told were multiple senior FBI officials.”

Other evidence is there too

In addition to the actual testimony of Comey and nine other senior federal law enforcement officials against the president, there is other related corroboratory evidence created as a result of those conversations. And this could bolster any potential obstruction of justice case against Trump.

There are Comey’s now-famous notes, which are careful, meticulous accounts of his meetings with the president. They are powerful not only for their detail but even for the atmospherics that tell a compelling story, according to people who have read portions of them.

Explaining why he took these notes, Comey told Congress: “I knew that there might come a day when I would need a record of what had happened, not just to defend myself but also to defend the FBI and our integrity as an institution and the independence of our investigative function. … [I]t was a combination of circumstances, subject matter, and the particular person.”

FBI agents and managers are inveterate note takers. It is part of the culture of the FBI. Several of the senior FBI managers Comey consulted with are also attorneys, who have similar traditions of memorializing important matters by taking careful and contemporaneous notes.

“That’s the culture of the FBI — you habitually document everything you do,” Lauren C. Anderson, a former senior FBI official who worked for the bureau for 29 years, told the New York Times, explaining why Comey made notes of his crucial conversations with the president. Her comments also would appear to explain why other senior FBI managers might have made similar sets of notes about their conversations with Comey.

Although it is unclear which FBI managers took notes and which did not, at least one person familiar with the matter said that James Baker, the FBI’s general counsel, made detailed notes of virtually every conversation with Comey or others about the Russia probe.

Those notes by Baker are crucial to investigators because Baker was a lively participant in discussions about whether to inform the Justice Department of the president’s pressure on Comey to end the Flynn investigation. During discussions about whether Comey or the Justice Department should give in to Trump’s request to say the investigation had not focused on him, Baker was the primary and strongest proponent that they not do so.

The potential testimony by Comey, McCabe, and so many other FBI witnesses could prove damning to Trump for other reasons. FBI agents and their managers are more than just highly credible witnesses. In the course of a typical FBI agent’s career, he or she works closely with federal prosecutors in making cases based on the testimony of witnesses first interviewed by the agent, and often testifies as a witness in cases, some dozens of times in the course of a career.

While most major governmental institutions have, according to most polls and surveys, faced some of their lowest ratings ever, the American public still retains strong confidence in its FBI. A November 2015 Pew Research national survey found that 68 percent of all Americans viewed the FBI favorably. Only four other federal agencies ranked higher: the US Postal Service, the National Park Service, the Centers for Disease Control and Prevention, and NASA.

Even Trump allies could hurt Trump

Comey testified to Congress that he shared with senior managers of the FBI the president’s efforts to thwart the bureau’s Russia investigation. But he did not inform the Justice Department of those efforts prior to Trump firing him. A major reason he didn’t do so, Comey said, was because the FBI’s leaders told him, “Look, it’s your word against the president’s. There’s no way to corroborate this.”

But Comey testified that during a private meeting with Sessions about another matter — “the president’s concerns about leaks” — he took the opportunity “to implore the attorney general to prevent any future direct communication between the president and me.” Comey told Sessions that leaving him alone with Trump “was inappropriate and should never happen again.” Comey said that Sessions “did not reply at all, his body language suggesting he was helpless or unwilling to do anything.”

Comey also testified that he expressed similar concerns to Rosenstein: “I explained my serious concern about the way in which the president is interacting, especially with the FBI.”

In his own testimony to Congress, Sessions sharply disputed Comey’s claim that he said or did nothing when Comey raised these concerns, saying he told Comey “that the FBI and Department of Justice needed to follow department policies regarding appropriate contact with the White House.”

But more importantly, while taking issue with that one aspect of the story, Sessions largely corroborated Comey’s account under oath — about how uncomfortable the then-FBI director felt with the president’s interactions with the FBI. Sessions is a Trump loyalist, the first US senator to endorse Trump, and the Trump administration’s attorney general — this only enhances his credibility as a witness whose testimony would harm Trump. (Of course, that relationship is now severely strained.) That Sessions recommended Comey’s firing as FBI director also, ironically, enhances his credibility as a corroboratory witness of Comey’s and against the president.

Rosenstein is yet to be heard from.