In The Spy Who Knew Too Much: An Ex-CIA Officer’s Quest Through a Legacy of Betrayal (Harper, June), Blum traces retired CIA officer Pete Bagley’s hunt to identify a mole within the agency.

What was the genesis for this book?

A CIA official told me about a story about a boat found floating on the Chesapeake Bay. They couldn’t find the boat’s owner. There were documents inside, and a burst transmitter, and he said it was one of the great mysteries of the CIA. I tried to answer what had happened to its owner, John Paisley, a high-ranking CIA official.

That led you to Bagley, who pursued evidence that Paisley was a traitor after he was forced out of the agency. What was the hardest part of his story to convey?

How can a man make the decision to go back to the world that he felt he’d successfully escaped from? Bagley felt that he owed it to the CIA to try to expose why things were going wrong, so it would perhaps awaken them to treason.

Is there anything you learned about how the KGB operated during the period you cover that suggests that a Russian mole might be at work today?

Everything I uncovered suggests that there is a continuum of treason that stretches from the early days of the Cold War to the present day. Through all these periods, the CIA has not only turned a blind eye to counterintelligence—the traitors in its midst—but also vilified those who hunted for moles. The agency’s security practices were so lax that Paisley was not polygraphed for over 20 years—and these cavalier practices exist today. There’s every reason to suspect that double agents are informing Moscow Center about U.S. intelligence activities in Ukraine. Bagley left the comfort of his retirement because he wanted to prevent bad things from happening. Unfortunately, for the nation and the agency, his investigations were scorned, dismissed as paranoia. That, apparently, was easier than admitting uncomfortable truths. And the nation is paying the price for this wrongheaded mindset today. We are in danger.

Has the CIA learned lessons from the Paisley case?

The unfortunate truth is that the lessons haven’t been learned. The agency, in fact, is still divided into two warring camps: those who refuse to believe that the Russians would dispatch KGB officers as deception agents—and those who saw through that program. It’s an intelligence catastrophe of national proportions. The mindset of the men who vilified Bagley is the controlling worldview of the agency today. It’s a narrow way of looking at the enemy.