Transcript for AG Bill Barr: Mueller's letter criticizing his summary of report 'a bit snitty'
word that attorney general William Barr has just now revealed that he will not testify tomorrow after fireworks at his testimony today. And the bombshell revelation that Robert Mueller, the special counsel, wrote him a letter to say he was unhappy with how Barr handled the Mueller report when it was turned in. Barr was asked today about that, why he did not reveal to the American people weeks ago that Mueller had sent him that letter, that they had talked about that Mueller was not happy. Barr answering those questions, and also today, calling Mueller's letter "A built snitty." ABC's Mary Bruce on the hill. Reporter: On capitol hill today, attorney general bill Barr forced to defend his actions against unprecedented criticism from Robert Mueller himself. Just hours before, news broke that Mueller had complained to the attorney general about Barr's decision to release his own summary of the Robert's conclusions. Mueller voiced his frustration in a letter to Barr on March 27th, writing that the attorney general's summary "Did not fully capture the context, nature, and substance of this office's work and conclusions." Adding, "There is now public confusion about critical aspects of the results of our investigation." This letter was an extraordinary act, a career prosecutor rebuking the attorney general of the United States, memorializing in writing, right? The letter is a bit snitty and I think it was probably written by one of his staff people. Reporter: Mueller wrote his own summaries of the report and twice asked Barr to release them. But Barr refused. The two men later talked on the phone and Barr insists Mueller did not accuse him of misrepresenting the report. There were notes taken on the call. May we have those notes? No. Why not? Why should you have them? Reporter: Barr knew of Mueller's concerns when he testified last month, but he didn't reveal them when asked about concerns from the special counsel's team. Members of the special counsel's team are frustrated at some level with the limited information included in your March 24th letter that it does not adequately or accurately, necessarily, portray the report's findings. Do you know what they are referencing with that? No, I don't. Reporter: Today, Democrats demanded to know why he didn't tell them about Mueller's letter. Why did you say you were not aware of concerns? Well, I answered a question. And the question was relating to unidentified members. I talked directly to Bob Mueller, not members of his team. Mr. Barr, I feel your answer was purposely misleading. Reporter: Barr called concerns over his summary "Mind-bendingly bizarre." And took this jab at Mueller. His work concluded when he sent his report to the attorney general. At that point, it was my baby. Reporter: Mueller did not make a determination on obstruction of justice. But Barr did. Declaring in his summary that the president did not obstruct. So, you accepted the report as the evidence? Yes. You did not question or look at the underlying evidence that supports the conclusions in the report? As the attorney general of the United States, you run the United States department of justice. If any U.S. Attorney's office around the country, the head of that office when being asked to make a critical decision about, in this case, the person who holds the highest office in the land, and whether or not that person committed a crime, would you accept them recommending a charging decision to you if they have not reviewed the evidence? Well, that's a question for Bob Mueller. Reporter: The president says he's exonerated, based on Barr's conclusions. But Barr insists he did not clear the president. No, I didn't exonerate. Reporter: Republicans pushed Barr on why Mueller didn't make his own determination. Tell me again, briefly, why Mr. Mueller told you he reached no conclusion. I really couldn't recapitulate it. It was unclear to us. Reporter: Democrats now want to hear from Mueller himself, but the Republican chairman of the committee wants today's hearing to be the last word. Why not call for Mueller to testify? Because I'm not going to do enough already. It's over. We did hear Lindsey graham today, Mary, say he would reach out to Bob Mueller and if he disagreed with what William Barr said today that he would invite Mueller's response. But in the meantime, Mary, as we reported, the attorney general was scheduled to testify tomorrow in the house, just moments ago, word is, he's now refusing to appear because of that format, there's been an argument over that for some time. And aside from Barr returning, the other person many want to see up on the hill answering questions is Robert Mueller. How likely is that? Reporter: Well, David, you heard Lindsey graham. He's made clear he's not going to call him in the senate. While over in the house, Jerry Nadler just announced that they're looking at may 15th for a potential hearing, but asked if Mueller has agreed, Nadler said not yet. But David, Barr has made clear he's not going to stand in the David? All right, Mary Bruce on the hill all day for us. Mary, thank you.
This transcript has been automatically generated and may not be 100% accurate.