Years ago, when Bob Mueller was a prosecutor in Boston, I asked him for some help.

I was chasing a story and trying to assess the credibility of some information other people in law enforcement had given me.

“No comment,’’ Bob ­Mueller said.

I said I understood and respected his position, then asked if he could help me on background.

“No comment,’’ Bob Mueller said.

“OK,’’ I replied, “but I just want to make sure I’m not going down the wrong road here, so could we talk off the record?’’

He looked at me and tilted his head.

“Off the record?’’ he asked.

I nodded.

“Off the record,’’ Bob Mueller said, “no comment.’’

Mueller survived 13 years as FBI director in a town that floats on leaks, but he was never a leaker. He tolerates the press, perhaps even believes a free press is essential to democracy, but he doesn’t leak.

And I suppose if your name is Donald Trump that part of Bob Mueller’s M.O. would bring you a degree of comfort. With Mueller as special counsel investigating any ties between the Trump campaign and Russia, President Trump will not die a thousand deaths from leaks about the investigation.

But the longer view is one that might give the White House considerably more pause.

Mueller’s appointment provided a rare instance of Democrats and Republicans agreeing on something. Mueller was universally praised as someone of integrity, as one of the nation’s most trustworthy and credible law enforcement officials.

According to those who have worked with Mueller, he is nonpartisan, someone who, as a prosecutor and FBI director, did not allow politics to influence his work.

But we all have biases, even if they’re unconscious biases, and Bob Mueller is favorably biased toward the FBI. He became FBI director a week before the Sept. 11 attacks and led the bureau’s transformation from an agency focused on fighting crime to one more engaged in counterterrorism.

Whenever the bureau was accused of falling short, Mueller mounted a full-throated defense of it. Whether the charge was that they failed to do enough when an Islamic extremist named Zacarias Moussaoui was arrested three weeks before the 9/11 attacks after he tried to learn how to fly a jumbo jet. Or that they dropped the ball after Russian officials tipped the FBI off about the extremist leanings of Boston Marathon bomber Tamerlan Tsarnaev.

He never explained or apologized for the FBI’s scandalous embrace of the murderous gangster Whitey Bulger.

“He may have a bias toward the bureau, but I don’t see that as a negative because he is scrupulously fair,’’ said Ed Davis, the former Boston police commissioner. “He is a law enforcement guy who will look at facts.’’

Facts have to be corroborated, and that’s why the White House should be worried.

After President Trump fired Mueller’s successor, Jim Comey, the president had some unkind words for Comey. Mueller wouldn’t recognize the president’s characterization of Comey as unprincipled.

Comey, then deputy attorney general, was next to Bob Mueller in 2004 when they went to the hospital bedside of Attorney General John Ashcroft to prevent White House officials from pressuring Ashcroft into reauthorizing a secret domestic surveillance program the Justice Department had deemed unconstitutional. Comey and Mueller threatened to resign if the program was reauthorized.

In 2007, that hospital room scene was the subject of a congressional hearing, at which Bush administration officials disputed Comey’s account. But contemporaneous notes that Mueller wrote after the hospital confrontation corroborated Comey’s version.

Ten years later, contemporaneous notes that Comey wrote after President Trump allegedly asked him to drop an investigation into former national security adviser Michael Flynn may become a critical piece of evidence in Mueller’s probe of Russian influence.

If, when all is said and done, this comes down to essentially the word of Trump and his White House against the word of Comey and what was his FBI, I have no doubt which side Bob Mueller would intuitively believe.

Kevin Cullen is a Globe columnist. He can be reached at cullen@globe.com