WASHINGTON >> Days after he was appointed special counsel, Robert Mueller visited the Capitol to meet with members of the Senate Judiciary Committee, which was conducting its own Russia investigation and needed to coordinate with his. Sen. Charles E. Grassley, R-Iowa and the committee’s chairman, came with a wisecrack.

You did well keeping the country safe as FBI director, he told Mueller at the end of the meeting, a senator in the room recalled. But you never answered mail from us, Grassley said of a Congress that conducts much of its business on paper.

Mueller laughed. Just keep sending those letters, he replied.

The lighthearted exchange hinted at a tension that has made Mueller a reluctant witness for two highly anticipated House hearings on the Russia investigation on Wednesday. Over decades of appearances before Congress, Mueller showed little patience for politics, and he grew weary of the partisanship that came with legislative oversight, according to interviews with former colleagues, law enforcement officials and lawmakers.

A review of dozens of hours of his hearings — Mueller has appeared before Congress 88 times dating back to 1990, according to the Senate Historical Office, among the most of any official ever — offers insight into what kind of witness he will be this week. He was by turns forbidding and protective of the FBI’s mission, yet sympathetic to Congress’ obligation to monitor the bureau’s transformation from a crime-fighting agency into a centerpiece of the government’s post-Sept. 11 counterterrorism apparatus.

Mueller brings a long-standing commitment to preparation to Wednesday’s hearings. He met into the evenings with FBI colleagues for days before congressional appearances, poring over thick binders in a large conference room next to the bureau director’s office on the seventh floor of the J. Edgar Hoover Building. Aides role-played as members of Congress who might have wanted to squabble with him on camera.

Before the special counsel investigation hearings, his old law firm, WilmerHale, has opened space for him at its offices in downtown Washington, said Robert T. Novick, a managing partner there. Another partner at the firm has functioned as Mueller’s representative in talks with Congress: Jonathan R. Yarowsky, the former general counsel for the House Judiciary Committee.

Mueller treated his appearances over the years with a kind of dread, said Lisa Monaco, his chief of staff at the FBI.

“I don’t think anybody loves going up there and sitting through hours of testimony and hours of speeches that maybe result in a question or not. He didn’t love it,” she said. “He would brace himself, because he knew it was an opportunity to put the FBI in the political cross hairs.”

Mueller was sworn in as FBI director a week before the Sept. 11 attacks. Lawmakers in both parties, unusually like-minded in response to the 2001 attacks, pressed him in several hearings on how the bureau missed leads on al-Qaida.

Mueller and Congress advocated the same broad overhaul of the FBI. Within a year, the FBI had reassigned 400 agents to counterterrorism from drug investigations, white-collar crime and other offenses. Mueller, who planned initially to hire 400 more analysts, knew how beholden he was to legislators for funding and direction. “We are not the policymakers,” he told an audience at Stanford University in 2002. “The FBI must use the tools that Congress gives us.”

The agency’s pursuit of what led to the Sept. 11 attacks rested, in turn, on Mueller. “He was the lead investigator of 9/11. And we were in a significant degree dependent on his ability and willingness to use his resources to get to the basic facts,” said former Sen. Bob Graham of Florida, the Democratic chairman of the Senate Intelligence Committee from 2001 to 2003.

Congress questioned whether Mueller could reorganize a bureau with tens of thousands of employees spread across the nation, many of whom were believed out of sync with Washington headquarters. Former Sen. John Edwards, D-N.C., planned to introduce legislation that would have removed the bureau’s domestic intelligence arm and created a new kind of superagency akin to Britain’s MI 5.

In a February 2003 hearing, a month after announcing a presidential campaign exploratory committee, Edwards used his five-minute question-and-answer allotment to criticize Mueller’s overhauls, reading off printouts of critical assessments and running out the clock before Mueller had a chance to respond.

“It will never be able to reform itself to do this job,” Edwards declared of the FBI.

Mueller fired back: Edwards had ignored the work the FBI had done for 17 months in connecting the dots.

“I have offered you an opportunity, personally, to come down to the bureau and be briefed on the changes that we have made since Sept. 11. You have declined to come down,” Mueller said, his voice shaking.

At the end of the exchange, Mueller turned off his microphone and stared icily at Edwards.

Mueller’s annual appearances in wood-paneled congressional office buildings were highly ritualized: oversight and budget hearings in both chambers of Congress and a threat assessment hearing with other intelligence agency chiefs. Loath to make small talk, he would sometimes skip the anteroom and use a side entrance, heading straight for the witness table, where he sipped on water or coffee. (Mueller, an early riser, was known among close FBI aides to be fond of Starbucks pumpkin spice lattes.)

He often deployed the kind of jargon that might be found in the IT manuals and organizational management books he kept in his FBI office: phrases like “predictive,” “analytical capability,” “the cyberarena,” “deliverables” and “exercisable options.”

By the mid-2000s, Mueller’s relationship with lawmakers soured in new ways. Democrats newly in charge of Congress were eager to cast the administration of President George W. Bush as criminal. Mueller was seen as an incorruptible exception, which made him something of a target.

He endured tough hearings after a Justice Department inspector general’s report in 2007 showed that the FBI had improperly used the Patriot Act to obtain information about people and businesses.

“Every time we turn around, there is another very serious failure on the part of the bureau,” Sen. Arlen Specter of Pennsylvania told Mueller during one such hearing.

During an exchange at an oversight hearing the next year about a classified program, Mueller said he believed that he did not owe senators more, as intelligence agencies were sufficiently briefing Congress. “That’s a classic nonanswer,” Specter thundered. “And I’ll let it stand for the record: You can’t do any worse than that.”

Even as Mueller’s appearances over his 12 years as director under Bush and President Barack Obama remained mostly respectful — members of Congress often praised his stewardship — they featured a regular amount of political preening that made Mueller deeply uncomfortable, his aides said.

“It’s inescapable. Every time he went up to the Hill, you saw that,” Monaco said. “Members saw the benefit of putting Mueller and the FBI between the poles of a debate, each side using the bureau or using him to try and score points.”

After Republicans took back control of the House in 2010, Mueller’s relationship with lawmakers turned more cynical. The mostly civilized panels he encountered gave way to more ornery hearings, particularly in front of the House Judiciary Committee, whose members will question Mueller on Wednesday.

In a May 2012 hearing with the committee, Rep. Louie Gohmert, R-Texas, told Mueller that the only reason he had been granted an extension to serve beyond the typical 10-year term as FBI director was because no one was on the House floor at the time to object.

A year later, Gohmert, who is still on the committee, accused the FBI director of failing to respond to a tip about a mosque that the Boston Marathon bombers had visited. “Your facts are not altogether well founded,” the typically reticent Mueller countered, explaining that agents had met with imams at the mosque.

On the same day, Mueller encountered Rep. Jim Jordan, an Ohio Republican who is now a close ally of President Donald Trump and one of the most vocal critics of the Russia investigation. He will also question Mueller again this week.

Jordan was furious. Why did Mueller not know the name of the lead agent in the FBI investigation of the IRS, which had been accused of targeting advocacy groups with “Tea Party” and “patriot” in their names? Jordan had been a regular guest on Fox News programs that devoted hours of airtime to the issue.

He continued to cut off Mueller, who closed his eyes in frustration. It was an active investigation, Mueller said repeatedly. He couldn’t say much.

“This has been the biggest story in the country, and you can’t even tell me who the lead investigator is?” Jordan asked.

Moments later, Mueller was grinning slightly. Jordan’s time was up.

“I’d be happy to take your questions in writing, sir,” Mueller said.

Weeks later, he retired.

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