The investigation into the origins of the FBI's 2016 Trump-Russia probe is reportedly zeroing in on two former top national security officials based on new information U.S. Attorney John Durham (pictured) discovered during a recent trip to Rome with Attorney General William Barr

The investigation into the origins of the FBI's 2016 Trump-Russia probe is reportedly zeroing in on two former top national security officials based on new information U.S. Attorney John Durham discovered during a recent trip to Rome with Attorney General William Barr.

Sources familiar with the investigation spearheaded by Durham told Fox News on Tuesday that the Connecticut attorney is 'very interested' in questioning former Director of National Intelligence James Clapper and former CIA Director John Brennan, an outspoken critic of President Donald Trump who recently called the probe 'bizarre'.

Durham and Barr have taken multiple covert trips overseas to collect evidence about the Russia investigation, including one to Italy where they reportedly heard a tape from a taped deposition with Joseph Mifsud, the professor widely credited with sparking the investigation by suggesting that Russia was colluding with the Trump campaign.

Mifsud allegedly offered to give ex-Trump aide George Papadopoulos Russian 'dirt' on Hillary Clinton in a move Papadopoulos has suggested was part of a set-up by intelligence agencies.

Sources familiar with the investigation spearheaded by Durham told Fox News on Tuesday that the Connecticut attorney is 'very interested' in questioning former Director of National Intelligence James Clapper (left) and former CIA Director John Brennan (right), an outspoken critic of President Donald Trump who recently called the probe 'bizarre'

Durham has ramped up his review in the last week by increasing his staff and his time frame with approval from Barr, who launched the probe at the conclusion of the Russia investigation earlier this year.

The expansion came amid concerns over whether the probe has legal or factual basis, multiple sources familiar with the proceedings told NBC News.

Two insiders claimed that there is tension between the Justice Department and the CIA over what classified documents should be submitted to Durham.

The U.S. attorney has reportedly added a number of current and former officials to his list of people to interview about how the Russia investigation was conducted.

Durham ramped up his review in the last week by increasing his staff and his time frame with approval from Barr (pictured), who launched the probe at the conclusion of the Russia investigation earlier this year

Brennan and Clapper, both Obama administration officials, were key players during the intelligence community's early assessment about potential Russian interference in the 2016 election.

They were both in office when the largely discredited Steele Dossier, which was compiled by former British spy Christopher Steele and alleged that Russia had damning information against Trump, was used to justify a secret warrant to surveil ex-Trump adviser Carter Page, one of the main targets in the Russia investigation.

Durham was initially appointed to review events leading up to the 2016 election and through Trump's inauguration in January 2017, but sources say his probe has been extended to include events through May 2017, when Special Counsel Robert Mueller took over the Russia investigation.

This week's developments came as Justice Department Inspector General Michael Horowitz is preparing to release the findings of his own investigation into the origins of the Russia probe.

Barr appointed Durham to conduct the separate investigation because he said he believed the IG inquiry would not be sufficient as Horowitz' efforts were limited to current officials in the Justice Department.

Republicans are reportedly hopeful that the findings of both investigations will bolster Trump's efforts against a House impeachment inquiry sparked by the president's dealings in the Ukraine.

On Tuesday a House GOP source told Fox: 'If the rumors are true that IG Horowitz’s report and findings in Durham's review will blast the conduct of the FBI’s Russia investigation, it will give Trump a lot of ammo to support his argument that he was unjustly targeted then and is being unjustly targeted now.

'It will justify Trump’s warnings about the Deep State acting to hobble his presidency.'