It's barely been a week since former FBI Deputy Director Andrew McCabe was unceremoniously fired (he was due to retire with full pension two days later), and already leaks about McCabe's efforts to discredit his political opponents in the Department of Justice are beginning to surface...

To wit, ABC News reported Wednesday that McCabe authorized an FBI investigation of his boss, Attorney General Jeff Sessions, over what Democrats have described as the AG's "lack of candor" during his confirmation hearing and his subsequent public testimony.

The federal criminal investigation was authorized "nearly a year" ago. McCabe's previously unreported decision took an extremely unusual step: Placing the AG in the crosshairs of the FBI, an agency Sessions is supposed to supervise.

Sessions was reportedly unaware of the investigation - and remained unaware when he fired McCabe on Friday. According to ABC's sources, only McCabe, Mueller, Rod Rosenstein - the deputy AG who appointed Robert Mueller to be special counsel - and a handful of top Democrats and Republicans were aware of the investigation. McCabe authorized the probe after receiving a letter from Democratic Senators Al Franken and Jim Leahy urging the DOJ to examine whether Sessions made false statements to lawmakers.

The probe ended shortly after Sessions met with Mueller in the fall. No charges were filed.

During his confirmation hearing early last year, Sessions said he hadn't met with any Russians and was unaware of any contacts between Russians and members of the Trump campaign. It was later reported that Sessions had met Russian ambassador Sergei Lavrov during a campaign event at Trump Tower. Much later, Mueller revealed that campaign advisor George Papadopoulos pleaded guilty to misleading investigators about contacts he had with Russian emissaries, with whom he was trying to orchestrate a meeting between Trump and Putin. Papadopoulos told Mueller that he informed Trump and Sessions about his efforts at a March meeting, which Sessions later said he did not recall.

"The Special Counsel's office has informed me that after interviewing the attorney general and conducting additional investigation, the attorney general is not under investigation for false statements or perjury in his confirmation hearing testimony and related written submissions to Congress," attorney Chuck Cooper told ABC News on Wednesday.

It's unclear how actively federal authorities pursued the matter in the months before Sessions' interview with Mueller’s investigators. It's also unclear whether the special counsel may still be pursuing other matters related to Sessions and statements he has made to Congress – or others – since his confirmation.

McCabe was fired after the DOJ's inspector general concluded that McCabe misled investigators about authorizing two agents to speak with reporters back in October 2016 about the bureau's investigation into Hillary Clinton to try and rebut reports that it had gone easy on Trump's rival.

When he was questioned later about that decision, McCabe "lacked candor – including under oath – on multiple occasions," Sessions said in a statement announcing McCabe's firing.

Of course, McCabe has denied this. And we wonder if he'd deny similar questions about the source of today's leak?

"The FBI expects every employee to adhere to the highest standards of honesty, integrity, and accountability," Sessions said. "As the [FBI's ethics office] stated, 'all FBI employees know that lacking candor under oath results in dismissal and that our integrity is our brand.'"

McCabe vehemently denies misleading investigators, saying in his own statement that he is "being singled out and treated this way because of the role I played, the actions I took, and the events I witnessed in the aftermath of the firing of James Comey."

The report is notable in that it's the first time that a senior Trump official still serving in his administration has been confirmed to be the target of a criminal investigation into perjury. Mueller has already proven more than willing to pursue perjury charges, and has secured guilty pleas for perjury from both former National Security Advisor Michael Flynn and Papdopoulos.

It begs the question: What other senior administration officials have been targeted by Mueller? And are any of those investigations still on going? Earlier, it was reported that Mueller was investigating the Trump camp's relationship with Cambridge Analytica, the designated pariah du jour...

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Interestingly, McCabe apparently made the same miscalculation as his longtime boss and friend James Comey: Namely, that launching an investigation against his superior (as Comey did when he oversaw the beginnings of what eventually became the Mueller investigation) would help insulate him...and just like with Comey, it didn't.