When Deputy Attorney General Rod Rosenstein appointed former FBI Director Bob Mueller as special counsel to investigate Russian election interference, I found myself neither surprised that Mueller had been tapped nor shocked when he accepted the call to duty. Like everyone else, I’d heard back in 2014 that he’d joined the Washington office of the law firm WilmerHale after departing the FBI. At that time, everything in my experience suggested to me that somewhere, somehow, he’d return to serve his country again. And after a few high-profile blips on the radar—most notably, his work overseeing the four-month investigation into the NFL’s conduct surrounding the Ray Rice assault incident—Rosenstein managed to bring Mueller back in.

The challenge Mueller agreed to tackle looked deep and wide: everything from the capability and intent of foreign governments and individuals, to the use and abuse of social media, to the labyrinth of international financial transactions. It was daunting, to say the least, even with the top-notch team of specialists he then gathered. My confidence nevertheless ran high that this complicated investigation would get the comprehensive treatment it demanded and be handled with the utmost integrity.

This trust in Mueller and his investigation didn’t come out of the blue. For more than a year, while serving as a CIA officer, I was his daily intelligence briefer in his role as director of the FBI. Five, often six, days a week I delivered to him the president’s daily brief (PDB) as well as voluminous other pieces of intelligence information and analytic assessments, primarily on terrorism.

Back in those days, the attorney general and the FBI director typically took their daily CIA report together, in the FBI’s Secure Information Operations Center (SIOC) conference room. At almost exactly the same time that I briefed them on the PDB, one of my colleagues did the same for President George W. Bush in the Oval Office. As my session with Attorney General John Ashcroft and Mueller ended, the two would proceed briskly down to the garage of the J. Edgar Hoover Building to get a brief ride up Pennsylvania Avenue to the White House—where, as the president’s briefer wrapped up his PDB session, the attorney general, FBI director, and homeland security adviser would enter the Oval Office. Bush would hear from each of them and lead daily discussions about how to use that day’s top secret information and analysis to protect the United States from additional attacks.

The relationship between daily intelligence briefers and their “customers,” as CIA officers for many decades have called senior policy makers, is a special one. The details of the briefing materials—including, but not limited to, information about intelligence sources and methods—and the sensitive conversations in that room remain sacred. I won’t discuss those things here, or anywhere.

But presenting complex information to Mueller, watching him digest it, answering his inevitable questions, and chatting with him and his staff on the margins of the sessions afforded me insight that I can appropriately share regarding his approach to complex problem sets—from L’Affaire Russe to Mueller’s personal style. This experience gave me confidence then about the fight against terrorism and the integrity of the Bureau under his watch, and it gives me confidence now in the work he is doing as special counsel.

What stood out to me most upon my starting the job, just months after 9/11, were Mueller’s attention to detail and his desire to understand how the CIA analysts arrived at their assessments. For a while, most of my briefings devolved into de facto intelligence hazing rituals. I discovered the hard way that when my presentation casually offered judgments lacking robust sourcing or logic, Mueller would ask me about the substantiation or argumentation until either my desperate searching through background materials could satisfy him or—more often in those first few months—I admitted that I’d have to get back to him after talking to the experts on that issue.

He wasn’t sending me down rabbit holes for the joy of doing so; he simply didn’t seem to trust analysis anchored to weak evidence or unclear reasoning. Inevitably, my follow-up on his questions resulted in either a quick nod of thanks or, particularly during the early briefings, another set of questions sending me back for more. Never did I feel that I’d been sent on a fishing expedition. Anything that initially appeared to be a tangent ended up having a purpose, usually to help him bring into focus one of the many pictures we were puzzling over.

In time, I came to appreciate the way he thought, and those interrogations went from constant, to often, to seldom. For months, I could only rarely foresee all of his questions on any given day. By the end of my briefing tour—after increasing my understanding of Mueller’s job, appreciating what the president was asking of him every day, and especially internalizing the way he processed information—I got up to about 90 percent over the course of a week. His passion for detail, however, meant that I could never anticipate everything on his mind six days in a row.

The president and his allies began arguing early in the special counsel’s investigation that ego and attention-seeking are driving Mueller in this probe. The man I got to know displayed the opposite characteristics. Our sessions—like the vast majority of top-tier intelligence briefings, both in my direct experience and across the history of the PDB—provided plenty of fodder for self-promotion if he had wanted to take them. He never did.

Without exposing anything classified, I can say that there were plenty of data points and assessments in my briefings about the impact of arrests, or other developments related to Bureau equities, that he could have gone to the press about. The overall briefings were Top Secret, codeword-controlled, dissemination-restricted—you name it. Yet I don’t recall a single episode where I saw the day’s news and thought, “Well, there he goes—blurting out something from the briefing to make him, or the FBI, look good.” That was not his modus operandi.

In one case, he shut down a media opportunity just as it was developing. I’d arrived in the SIOC early that day, as always, to prepare the materials (and myself) for effective use of every minute I had with him. Then an oddity occurred. Usually Mueller was on time, but I noticed that my scheduled start had just ticked by without the director’s entrance—a delay way outside his norm. I turned to the conference room entrance behind me just as I heard Mueller approaching. Seconds later, he marched through the door with his head down, looking at something on top of the binders he was carrying. He turned to walk behind me and take his usual seat, on my left.

Something else caught my eye: a boom microphone, looming over Mueller’s head, crossing into the conference room itself. Later, I learned he’d grudgingly agreed to allow some kind of “window into the director’s working day” film footage for one of the networks.

My quizzical look must have betrayed me, because he stopped dead in his tracks, looked me in the eye, and said, “TV crew. Should they come in for a minute?” With the PDB sitting there in front of me, everything in my training screamed “No!” All I remember mustering was a look of distress and a quick head shake communicating the same.

Mueller didn’t rebut. He didn’t insist that I close up the briefing books and cover my notes to allow the camera to enter what was, after all, his own conference room. Instead, he immediately reached back and shut the door in a smooth no-look move. The boom mic took a hit and remained outside. As Mueller took his place next to me, he returned my slight smirk, shrugged almost imperceptibly, and said, “Let’s get to it.” No showboating there.

So, what does this all suggest to me about Mueller’s approach to L’Affaire Russe and its related matters?

First, if anyone can manage this wide-ranging investigation, he can. It would strain credulity to believe that Mueller’s attention to detail and concern for logic and argumentation vanished when he left his long stint as FBI director.

Second, he won’t give up or back down. Then as now, his persistence in seeking information that he assesses he needs has opened new doors that he won’t shut prematurely.

Third, his clear disdain for unnecessary publicity or other extraneous noise serves him well. It hasn’t surprised me a bit that the spokesman for the special counsel’s office has remained the quietest voice in Washington.

My experience with Mueller suggests he will continue—and, ultimately, conclude—this investigation thoroughly, without distraction, and with integrity.