Acting FBI Director Andrew McCabe testifies before a Senate Intelligence Committee hearing on Capitol Hill in Washington, D.C., June 7, 2017. Aaron P. Bernstein | Reuters

A spokesperson for the FBI declined to comment for this story. The White House did not return calls seeking comment. Two attorneys recently hired by Mueller did not return phone calls either. On Wednesday, McCabe is scheduled to testify before a Senate committee. It is unclear whether he will testify about his private discussions with Comey. A senior law enforcement official told me that McCabe, Rybicki, and Baker now all consider themselves potential fact witnesses to Mueller's probe. Comey himself will testify a day later, before the Senate Intelligence Committee, where he is expected to recount his Oval Office encounter with Trump. It will be the first time Comey has spoken publicly of the matter. CBS, NBC, and ABC will air his testimony live. For many members of Congress, much of Mueller's new staff, and the American public, it will be the first time they hear Comey tell a detailed account of Trump pressuring him to shut down the FBI's investigation of Flynn over his contacts with Russia during and after the 2016 presidential campaign. Central to the special counsel's investigation will be whether Trump, in allegedly attempting to interfere with the Russia investigation, acted within the law or crossed the line and engaged in obstruction of justice.

The three officials in question are all lawyers — and, likely, note takers

It is unclear whether each of the officials Comey told of the encounter took notes on their conversations with the director about Trump. But the FBI's culture is one of creating contemporaneous notes and records, even on matters of far less importance. All three of the FBI officials are also lawyers, a profession known for its record keeping. Rybicki has had a long career in both the US Department of Justice and FBI. At the Justice Department, he worked for the deputy attorney general, the National Security Division, the Office of Intelligence Policy and Review, and the United States Attorney's Office for the Eastern District of Virginia, according to his official biography posted on the FBI's website. At both Justice and the FBI, he worked on both counterterrorism and counterintelligence matters, meaning that he was certainly read into the FBI's investigations of whether the Trump campaign or administration officials colluded with Russia to interfere on Trump's behalf to win a razor-thin election. Baker, too, has national security experience. From 2001 to 2007, he headed the Justice Department's office that would later formally become the department's National Security Division. An FBI press release about Baker's appointment as FBI general counsel states that while in that position, Baker "developed, coordinated, and implemented national security policy with regard to intelligence and counterintelligence matters for the department." The press release went on to note that Baker "provided the attorney general, the U.S. intelligence community, and the White House with legal and policy advice on a range of national security issues and conducted oversight of the intelligence community, including the FBI, on behalf of the attorney general." One senior law enforcement official familiar with the matter said that Comey specifically sought legal advice from Baker about when and how to tell the DOJ about Trump pressing Comey to shut down the Russia probe. The same official said that Comey and Baker had more than one discussion about the matter, and that Baker almost certainly made extensive notes about those deliberations. Both Comey and Baker sought the advice of Rybicki and McCabe as to whether to inform the Justice Department of Trump's pressure of Comey to shut down the Russia probe, according to this same official. All four of them had reservations about doing so because they did not fully trust Attorney General Jeff Sessions — and because the events were unprecedented in their experience. McCabe was deputy director under Comey. When Trump fired Comey on May 9, McCabe became the acting director of the FBI. A 20-year veteran of the bureau, McCabe began his career in the New York office, where he worked organized crime cases and was part of an FBI SWAT team. Later, he worked on counterterrorism, counterintelligence, and national security issues, experience that allowed him to become deeply immersed in the Russia probe. McCabe has testified to the Senate Intelligence Committee that as deputy director, he had "an oversight role over all of our FBI operational activity, including that [the Russia] investigation." The Trump administration said at one time that McCabe also was one of a number of potential candidates it was considering to replace Comey. He no longer appears to be a top candidate for the job.

One of the trio will speak to senators on Wednesday