“He's a great gentleman and a highly respected man,” Trumps told reporters as he left the White on Friday on a trip to Japan.
“They will be able to see how this hoax, how the hoax or witch hunt started and why it started. It was an attempted coup or an attempted takedown of the president of the United States. It should never have to happen to anybody else,” he said. "You are going to learn a lot. I hope it will be nice, but perhaps it won't be," he said.
"I want somebody that will be fair. I think William Barr is one of the most respected men doing what he does in our whole country. I just want him to be fair. I don't want him to be for me or anybody else. I want him to be fair. That's what he is. We are going to find what this yields," Trump continued.
“It's not payback,” he said. “I don't care about payback. I think it's very important for our country to find out what happened.”
He referred to long-standing efforts by House Republicans to get the nation's intelligence agencies to reveal what they knew about how the probe began -- an FBI probe that eventually became special counsel Robert Mueller's investigation.
"I'll tell you, declassifying. People have wanted me to do it a long time. It's very important to do. Basically what are we doing? We're exposing everything. We're being a word that you like, transparent. We're being transparent," the president said.
Trump's extraordinary move giving Barr broad authority to declassify information related to the investigation came in a memo made public Thursday night. It also directed the intelligence community to fully cooperate with Barr's review so that "all Americans learn the truth," according to a statement from White House press secretary Sarah Sanders.
"The Attorney General has also been delegated full and complete authority to declassify information pertaining to this investigation, in accordance with the long-established standards for handling classified information," according to the statement.
"Today’s action will help ensure that all Americans learn the truth about the events that occurred, and the actions that were taken, during the last Presidential election and will restore confidence in our public institutions," she said.
Barr has told Congress that there was "spying" on the Trump campaign, echoing repeated claims made by Trump as he fends off Democratic congressional investigations and calls for his impeachment.
"The first step is find out exactly what happened. And we're trying to get our arms around that getting all the relevant information from the various agencies and starting to talk to some of the people that have information," Barr told Fox News' Bill Hemmer last week.
"You know the thing that's interesting about this is that this was handled at a very senior level of these departments. It wasn't handled in the ordinary way that investigations or counterintelligence activities are conducted. It was sort of an ad hoc small group and most of these people are no longer with the FBI or the CIA or the other agencies involved," Barr said on Fox.
The Democratic chairman of the House Intelligence Committee, Rep. Adam Schiff of California, quickly attacked the move as political.
“While Trump stonewalls the public from learning the truth about his obstruction of justice, Trump and Barr conspire to weaponize law enforcement and classified information against their political enemies. The coverup has entered a new and dangerous phase. This is un-American,” Schiff said in a statement.
"This is a grotesque abuse of the intelligence community. And what's really disturbing is on the very same day that the president says I can't work with Congress to get anything done for the American people because I can't do that and respond to investigations at the same time, he initiates yet another investigation," House Judiciary Committee member Rep. David Cicilline, D-R.I., said on MSNBC Thursday night.
Giving Barr the power to declassify information is "really extraordinary." according to one former National Security Agency attorney.
“The problem is two-fold: First, as attorney general, Barr… is not naturally situated to have the full scope to know the impact of a declassification decision. He’s not in the best position to assess the potential damage," April Falcon Doss, a former NSA attorney and former Democratic counsel for the Senate’s Russia probe told ABC News.
"These sources and methods are fragile, perishable and the intelligence community agency who originated the information is best positioned to understand the impact of a particular declassification decision."
“Second, the AG is a political appointee. He’s already come under a lot of fire for behaving in ways that many have viewed as partisan. So, any decision he makes regarding declassification not only could be influenced by partisan considerations, but is almost certain to be seen as partisan considerations,” she told ABC News.
The review does not have any criminal implications, and would not involve criminal investigative power, according to one former U.S. Attorney.
"A criminal probe has as its target end criminal charges. A review, at least initially, does not anticipate criminal charges...," former U.S. Attorney Michael Stern told ABC News.
Stern agreed that it was dangerous "ordering one law enforcement agency to investigate another."
On Friday afternoon, Director of National Intelligence Dan Coats put out a statement saying the intelligence community would cooperate with Barr's review.
"I am confident that the Attorney General will work with the IC in accordance with the long-established standards to protect highly-sensitive classified information that, if publicly released, would put our national security at risk. The IC will continue to faithfully execute its mission of providing timely, apolitical intelligence to the President and policymakers," his statement said.
ABC News' Jordyn Phelps contributed to this report.