In the early 1990s, Robert Mueller had just left one of the most powerful posts in America's criminal justice system: assistant attorney general for the criminal division of the Department of Justice. Taking his experience into private practice, he was making well into six figures at a high-profile Washington law firm. Then he made a surprising call to Eric Holder, who was at the time the US attorney for Washington.

"He asked me if I could use a homicide prosecutor in my office," Holder recalled during a speech honoring Mueller, thinking that he was going to suggest a colleague. But to Holder's surprise Mueller was suggesting himself. Despite warning Mueller that he was significantly over-qualified and there would be no way to match his salary, Holder's response was quick: "When can you start?"

Soon after, Mueller, the future FBI director and special counsel would hit the streets, showing up at murder scenes, interviewing witnesses and making regular visits to the medical examiner's office. When his phone rang, he would bark "Mueller, homicide."

"It really was the equivalent of a two-star general retiring and then reenlisting as a second lieutenant to start his career all over again," said Mueller biographer Garrett Graff, author of "The Threat Matrix."

Mueller, now 74, began his Department of Justice career in 1976 as an assistant US attorney in San Francisco, and during the decades that followed took only two breaks to try out the private sector, each lasting no more than a couple of years.

The stints were so short-lived because of a simple fact, according to Graff: Mueller couldn't stand defending those he felt were guilty.

"He'll meet with the client, they'll explain the problem and he'll say 'Well, it sounds like you should go to jail then,'" Graff said. "There is not a lot of gray in Bob Mueller's worldview."

High-profile DOJ cases

That black-and-white outlook served Mueller well at the Department of Justice, where he oversaw some of the highest-profile cases of the last few decades including the prosecution of mobster John Gotti and Panamanian Dictator Manuel Noriega. But it was his investigation into the 1988 terrorist bombing of Pan Am Flight 103 over Lockerbie, Scotland that would most profoundly affect him.

"It was a very personal and a pivotal investigation of his career," according to Lisa Monaco, who served as Mueller's chief of staff when he was FBI director. "It is something that has stuck with him, and I think it was because he was so affected by walking the ground in Lockerbie after that plane went down, seeing the remnants of that plane, seeing the piecing together of the plane and the Christmas presents the passengers on that plane were carrying home to their family members, and seeing that all literally get pieced together in a warehouse in Scotland at the beginning of the investigation."

For years after the trial of the two Libyan terrorists, Mueller would quietly attend the annual December memorial service organized by the families, consoling those he had come to know well. When Scottish authorities announced in 2009 that they were releasing the one terrorist convicted in the case, Mueller was outraged.

"That did not sit well with him. He thought it was an injustice, a fundamental injustice for the families, and he did something very out-of-the-ordinary for him," Monaco said.

Mueller wrote a scathing letter to the Scottish authorities, saying in part, "your action makes a mockery of the grief of the families who lost their own. ... Where, I ask, is the justice?"

It was an unusual outpouring of emotion for a man who, according to those closest to him, regularly keeps to himself.

Tough boss

"He's a private man, he was very close to his family, he had a life outside the office, and we were there to work with him and work for him, and after that was over, time to go home," said Philip Mudd, a CNN counterterrorism analyst and the FBI's former senior intelligence adviser who met daily with Mueller.

At Mueller's annual holiday party at his home, Mudd recalled, Mueller would flick the lights on and off shortly after the end time noted on the invitation -- a not-so-subtle sign that it was time for his guests to leave.

In the office, Mueller was known as a taskmaster who worked long hours and demanded detailed briefings from his staff.

"Mueller is sort of notorious by reputation for being a pretty brutal cross-examiner. You know that's his prosecutor mindset, cross-examining people -- not just a hostile witness in a trial but also aides appearing before him in a briefing," Graff said.

"There's not a lot of back-and-forth, very quickly you're going to go through the details of the case, and you could do that in five or 10 minutes," Mudd said of his briefings with Mueller. It's a hard-hitting style that Mudd is convinced Mueller has brought with him to the special counsel investigation. "It's not like it's a professional choice, that's his DNA."

'Apolitical'

One topic never discussed, though, was politics. Serving with Mueller for more than four years, and attending thousands of meetings with him, Mudd said he never once heard him say anything political.

"Mueller is nonpartisan by almost any measure to a degree that almost no one in Washington actually is," Graff said.

Monaco, his former chief of staff, echoed that description: "He's apolitical. He's non-partisan. He is, as I think it has become quite clear, a pretty law-and-order guy."

It was one of the reasons President George W. Bush nominated him to be FBI director in 2001, noting during his introduction of Mueller that the FBI "must remain free of politics and uncompromising in its mission."

Now as Mueller testifies before Congress, after a two-year investigation and his 448-page report, the question is if he can continue remaining free of politics.