President Trump's nominee to be attorney general, William Barr, offered strong assurances about the special counsel's Russia investigation to the Senate panel that will be questioning him during his two days of confirmation hearings this week. Barr said in his prepared testimony, obtained by CBS News' Paula Reid, that he plans to allow special counsel Robert Mueller to complete the investigation into Russian meddling in the 2016 presidential election.

"I believe it is vitally important that the Special Counsel be allowed to complete his investigation," he wrote.

Referring to Mueller as a friend whom he has known for 30 years, Barr stated that he has confidence in Mueller's handling of the investigation.

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"I believe it is in the best interest of everyone – the President, Congress, and, most importantly, the American people – that this matter be resolved by allowing the Special Counsel to complete his work," Barr wrote, and he said that at this point he expects that the special counsel "is well along in his investigation."

Barr also promised transparency in dealing with the Russia investigation. He said he would follow the special counsel regulations "scrupulously and in good faith," and he suggested that the public and Congress would have some access to the special counsel's final report on the investigation. "I also believe it is very important that the public and Congress be informed of the results of the Special Counsel's work," Barr wrote.

The attorney general nominee also brought up a memo that has alarmed some Democrats who want to protect Mueller's investigation. In June, Barr wrote a memo to Deputy Attorney General Rod Rosenstein that was critical of Mueller's investigation. According to the memo, which the New York Times obtained, Barr argued that Mr. Trump should not be forced to submit to questioning by the special counsel, because "Mueller's obstruction theory is fatally misconceived," and is based "on a novel and insupportable reading of the law."

In his written testimony, though, Barr said that his memo had been "narrow in scope" and that it addressed only "a specific obstruction-of-justice theory under a single statute that I thought, based on media reports, the Special Counsel might be considering." But he said that the memo in no way questioned the core investigation and did not address other obstruction theories. He wrote that his memo did not say, "as some have erroneously suggested, that a President can never obstruct justice."

Barr has been studying every day since President Trump nominated him, a source familiar with his preparation tells CBS News' Paula Reid. To prepare for Tuesday's confirmation hearings, he has been working with a group of lawyers from the Office of Legal Policy. And though he's been through the confirmation process before, he held a mock hearing to practice.