Attorney General William Barr dropped a bombshell Wednesday, telling a group of senators that he believes spying against the Trump campaign did take place in 2016.
"I think spying on a political campaign is a big deal", he said.
"I think spying did occur", Barr said. "I think that is one of the principal roles of the attorney general".
He added he does not know why Mueller declined to review the four-page document Barr released about his report, except for wanting to have deniability.
President Donald Trump says he has spoken to the USA attorney general about tracing the origins of the inquiry that cleared him of colluding with Russian Federation.
"I feel that I have an obligation to ensure government power was not abused".
Barr said that after releasing a public version of Mueller's report - with some sections redacted - he will talk to leaders of the Senate and House Judiciary panels about providing them more information.
'I am concerned about it and I was asked about whether there was any basis for it, and I believe there is a basis for my concern but I'm not going to discuss the basis, ' he said.
"This is not launching an investigation of the FBI", he said.
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He falsely claimed that the Mueller report had found "no obstruction".
Attorney General William Barr, however, chose to not echo the president while he was delivering testimony before the Senate Appropriations Committee on Wednesday.
Republican Representative Robert Aderholt of Alabama asked Barr during Tuesday's hearing if the Justice Department is investigating "how it came to be that your agency used a salacious and unverified dossier as a predicate for FISA order on a US citizen?" A little while ago, he talked about the subject of spying on Donald Trump's 2016 campaign.
Although he was non-specific about what he was referring to by spying, Barr mentioned the intelligence community.
Barr told lawmakers at a congressional hearing Tuesday that he was reviewing "conduct" from around that period.
While there have been plenty of indicators that the Trump campaign was under surveillance, Barr's comments appear to be the closest to an official acknowledgement yet.
"That is one question I have", Barr said.
At least some of the information that underpinned the request came from a former British intelligence officer, Christopher Steele, who also was the author of the infamous Russian Federation dossier. The warrant was obtained based on evidence presented in the so-called "Steele Dossier", a piece of unverified and Democrat-funded opposition research that former FBI Director James Comey admitted he knew was unverified at the time it was presented to the FISA court.
Department of Justice Inspector General Michael Horowitz is now investigating whether or not federal investigators abused the Foreign Intelligence Surveillance Act, in order to get a warrant to investigate former Trump campaign adviser Carter Page.