Republican California Rep. Devin Nunes said there are three known groups that need to be examined to understand fully the origins of the Trump-Russia investigation.

During an interview Sunday on Fox News, the top Republican said the Justice Department, which is conducting a so-called investigation into the investigators, needs to determine the intersection of the FBI, Hillary Clinton's campaign and Fusion GPS, and a group of people at Cambridge University in London.

“Those are the three entities that we know," Nunes told host Maria Bartiromo on Sunday Morning Futures. "What the DOJ needs to get to the bottom of is, when did these all intersect? You don’t get that until you start to interview people and remember the Mueller team never bothered to interview many of these folks."

Former special counsel Robert Mueller concluded his investigation earlier this year and was unable to find sufficient evidence to establish criminal conspiracy between the Trump campaign and Russia. GOP investigators, such as Nunes, want to know when Mueller knew there was no coordination in his 22-month-long investigation. They also are concerned the FBI has not been forthcoming in its assertion that the counterintelligence into President Trump's campaign in July 2016, after Trump campaign adviser George Papadopoulos told an Australian diplomat Alexander Downer he heard the Russian has damaging information on Clinton.

Papadopoulos was allegedly told about this Clinton "dirt" by the mysterious Maltese academic Joseph Mifsud, whom Mueller admitted in his report had lied to investigators yet was never charged.

"Mifsud is at the heart of the investigation. Mifsud is the one who supposedly knows about Clinton’s emails — that the Russians have Clinton emails. He supposedly said this to Papadopoulos, something about emails," Nunes said.

Nunes said that the Mifsud-Papadopoulos-Downer communications were "the documented reason why they opened the investigation on July 31, 2016," but said that "it is clear" that the FBI was ramping up its inquiry earlier.

"What we’re trying to figure out is when did the FBI really start to run the investigation? What sort of processes did they use? What was the predicate?" Nunes asked. "It really appears like they were spying on the Trump campaign."

Nunes said another key issue for Attorney General William Barr and U.S. Attorney John Durham to examine in their review of the Russia investigation origins is the role played by the dossier written by British ex-spy Christopher Steele.

The dossier contained a litany of unverified claims about Trump's ties to Russia and was used extensively in the FBI's Foreign Intelligence Surveillance Act applications before the Foreign Intelligence Surveillance Court to justify surveillance of former Trump campaign associate Carter Page, an American citizen who was never charged with any wrongdoing. Steele compiled the dossier while he was hired by the opposition research firm Fusion GPS, which itself was being funded by Clinton's campaign and the Democratic National Committee.

While possible misuse of the Steele dossier in the FISA Court is the subject of an investigation by Justice Department Inspector General Michael Horowitz, whose report is expected to be released in September, the Barr-Durham review could follow up with prosecutorial action.

Stymieing his own efforts to inspect what the FBI was doing before July 2016, Nunes said the bureau has resisted handing over documents that he stresses he has a right to view under the purview of congressional oversight. Using his nickname for Mueller's team, the "Mueller dossier team," Nunes said the Justice Department "needs to look at these characters that were on Mueller’s team. I think they obstructed justice. They obstructed a congressional investigation."

Beyond the FBI, the Clinton campaign, and Fusion GPS, Nunes said "these intelligence-related folks" at Cambridge University need to be placed under scrutiny. He name-dropped Steven Schrage, an American citizen who helped organize a symposium in July 2016, which would be before the FBI's counterintelligence operation officially began. During Mueller's testimony last week, the California congressman asked the former special counsel to confirm whether his office did not investigate who was responsible for inviting Trump campaign associates Page and Stephen Miller to this symposium. Mueller declined to answer.

Nunes was by Bartiromo prompted to speak about efforts by possible U.S. informants to "entrap" people tied to Trump campaign, including a $10,000 payment to Papadopoulos in 2017 during Mueller's investigation just before he was arrested by the FBI, and contacts between Cambridge University figures and former national security adviser Michael Flynn that date back years before the 2016 election.

Back in 2014, Flynn met Russian-British graduate student Svetlana Lokhova during a trip to the University of Cambridge. Flynn was director of the Defense Intelligence Agency at the time. Reports came out in 2017 that said spy officials had concerns about Flynn's ties to Russians, including his encounter with Lokhova. Flynn's spokesman said the meeting was "incidental" and Lokhova told Fox News last year she is "not a Russia spy" and said she believes Flynn "was targeted and I was used to do it." The dinner at which they met was set up by a United Kingdom academic group that included Cambridge academic Stefan Halper, an FBI informant who was later sent to make contact with members of Trump's campaign.

Nunes said "none of this makes any sense" because Mueller's unredacted report does not touch on any of this, but has an intent focus on the infamous June 2016 Trump Tower meeting between members of Trump's inner circle, including his son Donald Trump Jr. and Natalia Veselnitskaya, an attorney with ties to the Kremlin who had predicated the meetup with the promise of dirt on Clinton.

Nunes said opposition research firm Fusion GPS had very suspicious activity surrounding the June 2016 meeting, including its co-founder Glenn Simpson meeting with Veselnitskaya multiple times. NBC News reported in November 2017 that Veselnitskaya said she received incriminating information on Democrats from Simpson, who had worked with her on a case involving money laundering by a Russian company. A lawyer for Fusion GPS said at the time the firm’s work on the Trump dossier was separate from the money laundering case and that the firm was unaware of the Trump Tower meeting when it happened.

But details on Fusion GPS was left out of Mueller's unredacted 448-page report, obliquely referenced only in a footnote. In fact, Mueller, who at times appeared to be unfamiliar with his own report, said during his testimony he was "not familiar" with Fusion GPS.

"This is nothing more, nothing less than a cover-up," Nunes said Sunday.