WATCH LIVE: Bill Barr faces questions about Mueller letter in testimony before Senate Judiciary Committee

Attorney General William Barr will testify starting Wednesday morning about special counsel Robert Mueller's report before the Senate Judiciary Committee.

The attorney general is expected to face tough questions about a letter Mueller sent to him in late March expressing frustration over his summary of the 22-month investigation, which examined at least 10 instances of possible obstruction of justice by President Donald Trump.

Sen. Charlie Crist (D-FL) asked Barr the last time he faced Congress, on April 9, about reports showing Mueller's team had voiced frustration about his summary letter, but the attorney general denied knowledge of those concerns -- despite having a letter about that from the special counsel himself.

The House Judiciary Committee has demanded a copy of the letter ahead of Barr's testimony in the Senate, which is scheduled to begin at 10 a.m.

Sen. Lindsey Graham (R-SC), a staunch defender of the president, chairs the Republican-controlled Senate committee.