Washington (CNN) As Attorney General William Barr reconstructs the early stages of the Russia investigation, he might discover that a lot of the answers he wants already exist.

In a letter this week to New York Rep. Jerry Nadler , chairman of the House Judiciary Committee, the Justice Department said that the review is "broad in scope" and will address "open questions" about "the activities of US and foreign intelligence services" as it related to the presidential campaigns in 2016.

But many of these topics have already been extensively addressed in a variety places, including the Mueller report , other US government reports, congressional testimony and public interviews.

Even so, critics of the Russia probe return to a similar set of facts that have already been reviewed, including hundreds of pages of documents that provide a deeper explanation of the origins of the investigation.

In all of it, there is scant evidence to support President Donald Trump's allegations that hostile and partisan forces inside the Obama-era Justice Department abused their powers to stymie his campaign.

And yet, in heeding Trump's public demand to "investigate the investigators," Barr has hinted that he has already uncovered improprieties, without offering any details. He's also stated outright that he believes Trump's campaign was spied on.

Barr has enlisted John Durham, a widely respected US attorney, to help him review the origins of the Russia probe and look for potential anti-Trump bias and alleged abuses of powerful surveillance tools.

These allegations form the core of Trump's conspiracy theories about the Russia probe. But many of them have already been debunked. While Trump has promoted his claims to the press, several of those he has accused of wrongdoing have rejected his theories under penalty of perjury.

Barr could certainly uncover new information, especially now that Trump has granted him extraordinary powers to declassify US intelligence materials. Barr says he'll "assemble all the existing information" from special counsel Robert Mueller, oversight inquiries on Capitol Hill, and the Justice Department's internal watchdog. But if he does that, he might find some answers that Trump doesn't like.

Origins of the Russia investigation

The President and his supporters have repeatedly insisted, despite substantial evidence to the contrary, that the Russia investigation began as a sham and was triggered by " the Steele dossier ," a series of memos by retired British spy Christopher Steele that accused Trump of colluding with Russia.

The memos were written during the campaign and many contain information about Russian meddling that pre-dated public acknowledgments from the US government about the Russian effort. In addition, some of the most salacious details of the dossier have not been verified.

The special counsel investigation and the Republican-led House Intelligence Committee inquiry both determined that the Russia probe was started because of Trump campaign adviser George Papadopoulos, contradicting Trump's unverified claims about the origins of the probe

Dueling memos from Democrats and Republicans on the House Intelligence Committee agree on this point. They were released in 2018 and derived from highly classified materials.

According to these investigations, Papadopoulos bragged to an Australian diplomat in May 2016 that the Russians had "dirt" on Democratic nominee Hillary Clinton and could help the Trump campaign. The Australian government informed the FBI two months later, after WikiLeaks started publishing internal emails that were hacked from the Democratic National Committee.

The Mueller report explicitly states that this information prompted the FBI on July 31, 2016, to open its investigation into whether the campaign was coordinating with Russia.

Both James Comey, who was FBI director at the time, and James Baker, who was the FBI's top lawyer, testified to Congress that the Russia probe was triggered by the Papadopoulos tip.

Even Rod Rosenstein, the former deputy attorney general who appointed Mueller as special counsel, has said the Russia investigation was "justified, and closing it was not an option."

Still, Barr could examine whether there was enough to trigger a full-blown FBI investigation. He also plans to "illuminate open questions" about "foreign intelligence services" that were involved in the Russia probe.

That could lead him down a path to key US allies including Australia, the UK and other European countries that share intelligence with the US . British and other European intelligence agencies provided the US with intercepted communications between Trump associates and Russians captured during routine surveillance of Russians, US congressional and law enforcement and US and European intelligence sources told CNN").

The New York Times reported on Wednesday that Barr is seeking interviews with senior CIA officers as part of his review.

Potential abuse of surveillance tools

Barr is also poised to pick up where Rep. Devin Nunes left off when it comes to probing alleged abuses of the Foreign Intelligence Surveillance Act, which gives federal law enforcement powers to wiretap US citizens and monitor their communications with court approval.

But much of Nunes' work has already been reviewed -- and rejected. This is what makes Barr's "spying" comments so controversial, since they indicate that he believes the FISA warrants may have been granted without sufficient evidence.

Last year, Nunes made waves when he released a memo that he said exposed how the FBI abused FISA laws to snoop on the Trump campaign. Trump embraced Nunes' findings. But when 412 pages of partially unredacted files about the FISA warrants were released, it became clear that Nunes' most nefarious allegations were nothing more than conspiracy fodder.

After the Nunes memo debacle, House Republicans launched their own inquiry, grilling top FBI and Justice Department officials behind closed doors about the FISA warrants. Those depositions failed to uncover evidence of a grand conspiracy against the President, and provided testimony to the merits of the investigation being opened. But the investigations have ensured coverage of Trump's claims continues.

Three top FBI lawyers involved in the FISA applications for Carter Page told lawmakers that proper procedures were followed. Baker, the FBI's former general counsel, said he personally checked to make sure that it would "adhere to the law and stand up over time." His deputy Trisha Anderson testified that they did not mislead the FISA court. FBI lawyer Sally Moyer said the team didn't even see it as a "close call."

"We would have gotten there on probable cause even without the Steele reporting," Moyer said.

Depths of the alleged conspiracy

Trump has used bellicose language to describe the Russia investigation, calling it an "attempted overthrow of the United States government" and repeatedly saying that people involved plot had committed "treason."

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Barr has fanned the flames while assuring the public that he's merely asking questions. He has invoked historical references to coups in Ancient Rome and says he wants to "carefully look at" whether top US officials tried to counter Trump. Barr told CBS News that "there is that tendency that they know better and that, you know, they're there to protect as guardians of the people."

Some of Trump's supporters have alleged , without proof, that the conspiracy went all the way up to President Barack Obama and the attorney general he appointed, Loretta Lynch.

But Lynch testified to lawmakers that she didn't even know about the Russia probe when it began. Neither did Bruce Ohr, the Justice Department official who has been dragged into the web because of his friendship with Steele and his wife's work for an opposition research firm.

At least five other key players testified under oath that there was no "deep state" conspiracy.

Strzok and Page

Perhaps the most notable members of the deep state conspiracy are former FBI agent Peter Strzok and former FBI lawyer Lisa Page, who were having an extramarital affair and exchanged anti-Trump text messages while holding important roles in the early months of the Russia probe.

"It's hard to read some of the texts ... and not feel that there was gross bias at work," Barr has said. But the Justice Department's watchdog examined their conduct , as it related to the Clinton email investigation, and said their texts looked bad but didn't directly affect their decision-making.

Barr's review could examine even more of their activities for bias. He might scrutinize their infamous "insurance policy" text, which Trump cites as proof of the conspiracy against him.

The private message in August 2016 was about early efforts to investigate Trump's campaign aides. Their message said, in part: "there's no way he gets elected -- but I'm afraid we can't take that risk. It's like an insurance policy in the unlikely event you die before you're 40."

Strzok tried to explain the context at a testy public hearing last year: The FBI was getting information about links between Trump aides and Russians. But Trump was far behind Clinton in the polls, and FBI officials were trying to decide how aggressively to follow the leads. Strzok said his view was that "we need to do our job" so the threats would be assessed if Trump won. The FBI was simultaneously investigating Clinton's use of a private server.

Page offered the same explanation when she was grilled behind closed doors on Capitol Hill. Her testimony lasted two days and Republicans later released 370 pages of transcripts

"This is an extraordinarily conservative organization," Page said of the FBI. "So, the notion that there's a deep state conspiracy about anything is laughable."

So far, Bill Barr isn't laughing.