Josh Campbell is a CNN analyst covering national security issues. He previously served as a Supervisory Special Agent with the FBI, special assistant to the bureau's director, including Jim Comey, and is writing a book on recent attempts by elected officials to undermine the rule of law. Follow him on Twitter at @joshscampbell . The views expressed in this commentary are his own. View more opinion on CNN.

(CNN) The FBI is back in the middle of a political storm, with a new tell-all book by fired deputy director Andrew McCabe. According to an interview with CBS "60 Minutes," the former lawman reveals in his book that discussions were underway at the Justice Department to possibly recruit cabinet officials and invoke the 25th Amendment to remove President Trump after he terminated then-FBI director James Comey.

The book has already riled Trump, who quickly blasted the former deputy director as a "disgrace." A White House spokesperson called McCabe "an embarrassment," and the administration is no doubt gearing up to refute any other damning revelations McCabe might make.

But with so many people involved in the book now caught lying, how are we to make sense of things?

President Donald Trump has a documented pattern of lies. His spokesperson, White House Press Secretary Sarah Sanders, has a fleeting relationship with the truth. And McCabe, a familiar target of attack, was fired for lying to government investigators.

Full disclosure, I worked with McCabe at the FBI and briefly served as his special assistant when I was a career agent. As someone who was fortunate enough to know him up close, he is without question a good man who served his country for over two decades as he worked to catch criminals and stop terrorist threats. Rather than choosing to make money by building skyscrapers, McCabe dedicated his career to building cases and serving the public by locking up criminals who violated the law.

Certainly, his past record of service does not excuse or even minimize the allegations he lied to the Justice Department during a leak investigation. McCabe has denied the allegations against him, but I know from experience that the reputation of the current inspector general is unimpeachable. Despite this, it is important the public understands the full nature of the person who continues to be the subject of a political smear campaign by the President.

The White House will insist McCabe's alleged lies to the inspector general mean we cannot trust anything he says. But that's pure politics.

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In my judgment, the most productive way to look at his situation is through the lens of an FBI agent. When someone whose integrity has been called into question wants you to believe something, ask yourself if what they are saying squares with other things you know.

When I was an investigator, I interviewed numerous people whose reputations were less than stellar. This didn't mean everything they said to me was false. Instead, it meant I had to find some way to corroborate what they were saying and compare their claims to other things I knew to be true.

For example, McCabe writes that the President acted like a bully and at one point disparaged his wife, who ran for public office and lost, as a "loser." He also compares Trump's actions to that of a mob boss.

We know from covering this President that he frequently bullies and degrades people. His Twitter feed is a repository of caustic insults. He has called people "dogs" and has routinely insulted the intelligence of his enemies, whom he has claimed have a "low IQ." Many experts also look at his intimidation tactics used against government witnesses and his criticism of cooperators as "flippers" as reminiscent of a mafia kingpin.

Put simply, we can rest assured that at least some of McCabe's version of events is true because we have other information we can use to inform our judgment.

That's not to say that we won't encounter parts of McCabe's book involving Trump that simply cannot be corroborated by outside experience.

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We know McCabe took contemporaneous notes after his interactions with the President, which likely serve as the basis for his recollection, but Trump will insist they are filled with lies. We can also understand that anyone in McCabe's position -- fired just before he was eligible to retire -- probably has some scores to settle.

In those instances where we are faced with the proverbial he said-he said scenario, we will simply have to judge for ourselves whose record, character and credibility we trust the most.