For almost 2 years, Robert Mueller led the special counsel investigation into Russian election interference — and whether the Trump campaign coordinated with the Kremlin.

Previously, he was the longest-serving FBI director since J. Edgar Hoover. Mueller was nominated to the position by President George W. Bush, and sworn in a week before 9/11.

On May 17, 2017, former Deputy Attorney General Rod Rosenstein appointed Mueller special counsel to take over the Russia investigation, and on May 29, 2019, Mueller announced he was stepping down following the release of his final report in April 2019.

Now, Mueller is back in the spotlight to testify to Congress about the report. Here's a look at Mueller's career, and how he got to where he is now.

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The special counsel Robert Mueller, 74, announced in a May press conference that he was officially stepping down after the release of his full, redacted report on the Russia investigation to the public in April.

He'll return to the spotlight on Wednesday to testify before two House committees.

The Russia probe was also examining whether the Trump campaign colluded with the Kremlin, and if President Donald Trump has obstructed justice while in office.

In Washington, Mueller has a reputation for being a tenacious investigator. Both Republicans and Democrats welcomed his appointment in May 2017 with bipartisan backing.

His team indicted dozens of people, companies, and entities in the investigation, but the breadth and length of his investigation irked many Trump supporters, and the president himself.

Read more: Mueller says he's resigning from the Justice Department and officially closing the Russia investigation

Mueller made it explicitly clear that the report did not exonerate Trump, and that his office had no ability to charge Trump with a crime given existing DOJ policy prohibiting prosecutors from indicting a sitting president.

"If we had had confidence that the president clearly did not commit a crime, we would have said so," Mueller said in his only press conference.

Now that Mueller's work is done, it is up to Congress to determine whether or not to initiate impeachment proceedings against Trump. With Mueller set to testify before Congress, see what you need to know about him.