The revelation of a possible third scope memo for special counsel Robert Mueller showed Deputy Attorney General Rod Rosenstein was "scared to death" of top FBI brass, said attorney Joe DiGenova.

Last week, the Washington Examiner's Byron York revealed the existence of a third scope memo, which remains shrouded in secrecy but indicates the instructions Rosenstein gave to Mueller for his Russia investigation were more extensive than previously known.

Reacting to the report on Fox Business, DiGenova, a former U.S. attorney for the District of Columbia, said it contributes to the notion that the Russia investigation was set up on "illegitimate" grounds and stems back to Rosenstein, who appointed Mueller as special counsel.

"When history of this scandal is written, one person will be blamed and should be blamed for the entire wasting of two years of President Trump's presidency. Rod Rosenstein was a coward," DiGenova told host Lou Dobbs.

DiGenova claimed Rosenstein was frightened of FBI Deputy Director Andrew McCabe, who from May to early August 2017 led the bureau after President Trump fired Director James Comey and Attorney General Jeff Sessions recused himself from the Russia inquiry.

"He didn't know how to tell McCabe and those people to stop fooling around and messing around with Trump," DiGenova said. "He came in, he found out about the Hillary Clinton investigation. He saw how rabidly anti-Trump they were, and he, Rosenstein, caved and gave up the power of the attorney general and the deputy attorney general to a bunch of corrupt FBI officials. And that third scope memo is proof that he was scared to death of McCabe."

DiGenova served as an independent counsel in the 1990s for a case on President Bill Clinton's passport before he was elected. Last year, it was announced DiGenova and his wife Victoria Toensing were joining Trump's legal team for the federal Russia investigation, but that plan was nixed within days. He has been highly critical of Mueller's Russia investigation, claiming that Trump had been "framed" by the Justice Department and the FBI.

Rosenstein left the DOJ last month after Mueller completed his 22-month-long investigation. McCabe was fired from the FBI on March 16, 2018, after the Justice Department's Office of the Inspector General determined he misled investigators about the role he had in leaking information to the Wall Street Journal in October 2016 about the investigation into the Clinton Foundation. McCabe, who acknowledged he ordered the obstruction of justice investigation into Trump, argued that his firing was an attempt to discredit the FBI and Mueller's investigation. He has since called for House Democrats to begin impeachment proceedings against the president.

Trump's allies have applauded Attorney General William Barr for his efforts to investigate the origins of the Russia investigation, but although Rosenstein, McCabe, Comey, and other officials they often accused of harboring bias against the president are out of government, some conflict remains. FBI Director Christopher Wray took heat after he disagreed with Trump over whether there was improper spying on his 2016 campaign. In an interview with the Hill on Monday, Trump declined to say if he had confidence in Wray. However, on Tuesday he appeared to walk that back by telling reporters that "yes" he is confident in Wray's performance.

Mueller's report was released with redactions in April, but fascination over the unseen aspects of his work remain intense. Republicans have called the case closed while Democrats argue Mueller left it to them to take up the investigation due to the limitations of his mandate, including indicting or recommending charges for obstruction of justice.

The first scope memorandum, which was immediately made public, was written by Rosenstein in May 2017 when Rosenstein first appointed Mueller as special counsel to investigate Russian interference in the 2016 election. Rosenstein said Mueller would look at any links or coordination between the Russian government and individuals associated with the campaign of Trump, any matters that arose directly from the investigation, and anything else within the scope of the DOJ’s special counsel regulations.

A second scope memo was issued in August 2017, in which Rosenstein wrote it provided "a more specific description of your authority." A heavily redacted version of the August memo has been released to the public, which revealed no specific assignments other than Mueller being authorized to investigate former Trump campaign manager Paul Manafort for possible conspiracy with Russia and payments he received from the Ukrainian government as a lobbyist.

York wrote that although the contents of the third memo, dated Oct. 20, 2017, remain secret, its existence suggests something was going on behind the scenes in the relationship of Mueller and his supervisors at the Justice Department.

One of the lawmakers to see it, House Intelligence Committee ranking member Devin Nunes, told Fox News that it looked to him to be a "CYA [cover your ass] memo.” The California Republican said the Mueller report “talk[ed] about this expanded investigation and they list[ed] a bunch of names in there.”

“But it looks to me like they wanted a reason — there must’ve been a legal reason why they needed an addendum to this,” Nunes said. “Because my guess is that they expanded this so far and wide.”