Robert Mueller testifies

Former special counsel Robert Mueller received some advice from the Department of Justice ahead of his congressional hearing today.

The Justice Department wrote a letter to Mueller on Monday that said he must adhere to the 448-page report he issued earlier this year.

The letter, sent in response to Mueller's request for guidance about his testimony, is signed by Associate Deputy Attorney General Bradley Weinsheimer.

In the letter, Weinsheimer quotes from Mueller's public statement in May where he indicated his reluctance to testify, and notes that the Justice Department does not typically allow prosecutors to appear before Congress and discuss their cases.

Weinsheimer told Mueller that DOJ policy prevents him from commenting on the legal conclusions his office made "with respect to uncharged individuals, other than information contained within the portions of your report that already have been made public." He also said Mueller should not testify about portions of the public report that have been redacted or about uncharged third parties.

But, not everyone agrees with the DOJ's advice: In an interview with CNN's Alisyn Camerota, House Judiciary Chairman Jerry Nadler said it was arrogant for the Justice Department to send a letter about what Mueller can say when he testifies in front of Congress.

He went on to say Mueller does not need to comply with the letter.

Former special counsel Robert Mueller has decades of experience testifying before Congress, but the last time he did it was more than six years ago during his final weeks serving as FBI director.

Here's a look at Mueller's career:

Years of legal work: Mueller 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: Mueller couldn't stand defending those he felt were guilty, Mueller biographer Garrett Graff, author of "The Threat Matrix."

He had high-profile DOJ cases: Mueller 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.

Mueller was a tough boss: In the office, Mueller was known as a taskmaster who worked long hours and demanded detailed briefings from his staff. At Mueller's annual holiday party at his home, Mudd, a CNN counterterrorism analyst and the FBI's former senior intelligence adviser who met daily with Mueller, recalled that 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.

He's known for being "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. 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."

Watch more about Mueller's career:

Former special counsel Robert Mueller will testify before the House Judiciary and Intelligence panels this morning beginning at 8:30 a.m. ET.

The former special counsel has been a reluctant witness, and is appearing before subpoena. Democrats plan to push Mueller on several aspects of his investigation as well as the question if he would recommend charges against President Trump were he not the President.

Here's what we know about today's hearings: