Attorney General William Barr faces tough new questions about his response to special counsel Robert Mueller's report at a Senate hearing on Wednesday. Judiciary committee members want to know why Mueller told the attorney general he was frustrated with Barr's original summary of the report. In a letter, Mueller told Barr his message to Congress "did not fully capture the context, nature, and substance" of Mueller's views on obstruction of justice.

Barr previously told lawmakers he didn't know if Mueller supported his conclusion on obstruction. CBS News has confirmed that not only did Mueller send Barr a letter complaining about the way Barr described the special counsel's findings, but the two men also spoke on the phone. During that conversation, Mueller asked for additional information to be released, but the attorney general only promised to release the full report as soon as possible, reports CBS News correspondent Paula Reid.

Almost two weeks after his call with Mueller, Barr denied knowing where Mueller stood on his characterization of the investigation's findings.

"Did Bob Mueller support your conclusion?" Sen. Chris Van Hollen of Maryland asked on April 10 before a congressional committee.

"I don't know whether Bob Mueller supported my conclusion," Barr responded.

Barr has publicly defended his decision not to charge the president with obstruction of justice.

"The evidence developed by the special counsel is not sufficient to establish that the president committed an obstruction-of-justice offense," Barr said April 18.

In the letter first reported by the Washington Post, Mueller wrote to Barr: "There is now public confusion about critical aspects of the results of our investigation. This threatens to undermine a central purpose for which the department appointed the special counsel: to assure full public confidence in the outcome of the investigations." Mueller also requested that Barr release the report's introductions and executive summaries to the public.

According to the Justice Department, after receiving the letter, Barr called Mueller, his longtime friend and colleague. The attorney general expressed that he did not want to release the report in piecemeal fashion. The two men agreed to get the full report out with necessary redactions as soon as possible.

The Justice Department points out Mueller also said, "Nothing in the attorney general's March 24 letter was inaccurate or misleading."

We still haven't heard from Mueller directly, and it took more than three weeks for the Justice Department to release the report. During that time the president used Barr's summary to characterize the report's findings.

"It was proven very strongly no collusion, no obstruction, no nothing," Mr. Trump said March 26.

"Frankly, there was no collusion and there was no obstruction," he also said April 11.

CBS News has obtained a copy of Barr's prepared remarks. There is no mention of the letter from Mueller or his conversation with him, but he says nobody at the Justice Department ever overruled the special counsel. He plans to use his opening statement to double down on his decision not to charge the president with obstruction of justice even though Mueller could not come to a decision and laid out evidence outside on both sides of the issue. This is what Barr plans to say, but as we have seen, Barr has a tendency to go off script.