As the daughter of an F.B.I. agent and a third-generation federal prosecutor, I grew up believing that the men and women of the F.B.I. were always the good guys, combating crime and working hard toward the ultimate goal of keeping us all safe. My father was that kind of F.B.I. agent, and so were the people I worked with.

But all the while my father and other F.B.I. agents were working to keep Americans safe, F.B.I. special agent Robert Hanssen was working just as hard to betray the agency he had sworn allegiance to and the people and country he had sworn to protect.

For 20 years beginning in the early 1980s, Hanssen sold national secrets to the Russians, beginning with the betrayal of an F.B.I. double agent for just $100,000. The dollar amount of the damage he caused has been put at $10 billion, but putting a price on the lives he destroyed is incalculable.

His actions cost lives and gutted operations critical to U.S. security. In the process, he also besmirched the reputation of the F.B.I. and branded it in ways that echo still—as an agency under siege, one unable to regulate itself from within.

After two decades of selling America out to the Soviets and, later, Russia, Robert Hanssen was finally arrested on February 18, 2001. He will remain incarcerated in federal custody for the remainder of his years. (Per his plea agreement with the government, neither Hanssen nor his wife, Bonnie, is allowed to speak with the press.)

F.B.I. agents arrest Hanssen near his home in Virginia, 2001.

In my interviews with present and past F.B.I. and C.I.A. agents, I asked the same question: “Could there be another Hanssen today?” Their resounding response was “Yes.”

Present tensions between the U.S. and Russia are at a level not seen since the Cold War, and the stakes are higher than they’ve ever been. In Hanssen’s day, most records were still analog. Today, with the interconnected nature of records and cyber-security, access and data breaches could have exponential repercussions.

With the history of Hanssen in the rearview mirror, the F.B.I. has implemented protective measures such as mandatory polygraph exams and financial disclosures for all agents. But these measures are not enough, according to my high-level intelligence sources, to prevent another breach. Agents who might have financial problems or are discontent with their career advancement, or are simply looking for a thrill to perk up their bureaucracy-bound lives, will give in to their own worst instincts, the dark angel of their psyche.

When that line gets crossed, you can institute all the polygraphs you want, run all the credit checks imaginable, but there’s always going to be the one agent with a high security clearance who slips through the net and sits down in the basement to write a letter offering his services to the Russian intelligence chief stationed in Washington.

Lis Wiehl is a former federal prosecutor and the author of several books. Her latest, A Spy in Plain Sight: The Inside Story of the FBI and Robert Hanssen—America’s Most Damaging Russian Spy, is out now from Pegasus