Barr first confirmed publicly before Congress on April 9 that he was launching a separate investigation from the ongoing Justice Department inspector general probe into whether there was any improper or unauthorized surveillance of members of the Trump campaign prior to Trump's inauguration.
Democrats have sought to label Barr's inquiry as a "fishing expedition," intended to provide cover for Trump's allegations that he was personally victimized by intelligence officials in the Obama Administration in an "attempted coup."
When asked by Sen. Kamala Harris, D-Calif., on May 1 whether he had ever been pressured by President Trump or anyone in the White House to open an investigation into anyone, Barr initially answered he was, "trying to grapple with the word 'suggest.' "
He then added, “There have been discussions of, of matters out there that, uh — they have not asked me to open an investigation."
Since the early days of his presidency, Trump, along with several of his Republican allies in Congress, has raised suspicions of a 'deep state' conspiracy to undermine his electoral victory, and made open calls for the prosecution of intelligence officials involved in the early stages of the Russia investigation.
Last week, FBI Director Wray said he had personally seen no evidence of improper surveillance and that he was cooperating with Barr to "get to the bottom of an understanding of what the circumstances surrounding the initiation of that investigation were at the department and the FBI back in 2016."
A source familiar with the matter confirmed to ABC News Tuesday that CIA Director Gina Haspel and Director of National Intelligence Dan Coats are also cooperating with Barr’s investigation.
While Barr has said he has "concerns" there was potential improper "spying" on members of the Trump campaign, he also has acknowledged he has seen no evidence to back up allegations by the president and his allies of unauthorized surveillance.
When asked in his congressional testimony last week about Barr's use of the term "spying," Wray answered that it was not a term he would use.
"I didn't understand his answer because I certainly thought the attorney general answered it perfectly," Trump told reporters Tuesday. "I thought it was a ridiculous answer."
Trump declined to answer whether he still has confidence in Wray's leadership of the FBI.
In a speech in Baltimore Monday evening, former deputy attorney general Rod Rosenstein said he believed the investigation "was justified, and closing it was not an option."
"If the inspector general finds significant new facts, I would reconsider my opinion," Rosenstein said. "But I always need to base my opinions on credible evidence."
Durham, who was unanimously confirmed by the Senate as U.S. attorney in 2018, has a storied history of investigating issues of corruption and national security under both Democratic and Republican administrations.
Former Obama Attorney General Eric Holder in 2009 tasked Durham to investigate whether the CIA destroyed tapes of interrogations and abused prisoners in its custody. Prior to that, he had been appointed by Former Clinton Attorney General Janet Reno to probe allegations of law-enforcement corruption in Boston.
The U.S. Attorney's Office for the District of Connecticut declined to comment on Durham's appointment.