A veteran spy catcher who helped bring down one of the most notorious Russian spies in American history thinks a mole is burrowed in the U.S. government.

Eric O’Neill worked as a young FBI investigator to catch Robert Hanssen, who held key counterintelligence positions in the bureau while spying for years for Russia. He was arrested in 2001 and died in prison last year. Mr. O’Neill told The Washington Times that spy operations inside the U.S. government are still a threat.

“There probably is somebody else right now somewhere in a government agency giving up secrets,” Mr. O’Neill said in an interview at the Black Hat USA conference in Las Vegas last week.

---

---

Asked whether such a spy has been in the government since Hanssen’s time at the FBI, Mr. O’Neill hesitated while cybersecurity professionals and hackers shuffled by at the Mandalay Bay resort.

“Maybe was, no, I’m not sure,” Mr. O’Neill said. “Not at that level. I don’t think we’re ever going to … have another Aldrich Ames, Hanssen level because it’s harder now. But there will always be spots.”

Like Hanssen, Ames betrayed the CIA to spy for Russia during the height of the Cold War. Ames was arrested in 1994, pleaded guilty and was sentenced to life in prison.

In the ensuing decades, rumors have swirled about a fourth high-ranking Russian spy inside the U.S. government alongside Ames, Hanssen and Edward Lee Howard, a CIA officer who defected to Moscow. Howard reportedly died after a fall at his Russian home in 2002.

Investigations searching for that fourth Russian spy have produced little. Former CIA officer Robert Baer published a 2022 book detailing efforts to uncover a purported “fourth man” that identified a potential suspect who disputed allegations in the book.

Mr. O’Neill, tasked with surveilling Hanssen as his assistant, does not accept that unfinished business has been lingering since the investigation. Mr. O’Neill’s intimate investigative work brought him as close to the turncoat as anyone and was dramatized in the 2007 film “Breach.”

He said many years of interrogations have made sure the U.S. government knows the extent of the secrets that Hanssen shared with Moscow.

“The FBI learned a lot from catching Hanssen and was able to close a lot of the doors that allowed a trusted insider like that to work internally,” he said. “It’s also a bit harder now because, in the height of his career, he was, one, stealing paper — not a lot of that anymore — and, two, he was stealing from computer systems [that] the FBI just never even considered a trusted insider would steal.”

Now, however, the U.S. government has plenty of reason to think it has a counterintelligence problem.

Earlier this month, the Biden administration needed to rely on a handful of foreign nations to complete the largest prisoner exchange since the fall of the Soviet Union. Amid a dearth of outed Russian spies in American prisons, the U.S. government sweetened the swap with the aid of Russians held by Germany, Norway, Poland and Slovenia.

Near the end of the Trump administration, government officials began acknowledging new counterintelligence problems and started making changes.

William R. Evanina, head of U.S. counterintelligence, said in 2020 that 2019 was a “horrible year” for the intelligence community, with secrets spilling from America, particularly via the private sector. Mr. Evanina announced a new counterintelligence strategy shortly before the COVID-19 outbreak spread to America and upended life around the world.

In 2021, the Pentagon adopted a new approach to countering foreign spies. A nominee for a critical Department of Defense post revealed last month that the strategy meant adopting a “more offensive posture, accepting greater risk” to confront advanced persistent foreign intelligence threats to America’s military.

Mr. O’Neill said the more aggressive shift is smart. He touted “old-school counterintelligence” that does not wait for threats to emerge and enabling the work he and others conducted undercover for the FBI as investigative specialists, better known as “ghosts.”

This year, though, foreign adversaries’ espionage goals are accomplished not only face-to-face but also screen-to-screen.

Mr. O’Neill, who left the FBI in 2001, has worked as a cybersecurity consultant and acknowledged that he remains worried about virtual trusted insiders breaching digital defenses.

As he has helped grow the technology startup NeXasure, he said he frets about how much easier and cheaper it is for adversaries to steal someone’s digital credentials compared with the logistics involved in physically going undercover and recruiting spies.

“You don’t even have to leave Moscow anymore. Espionage has changed,” Mr. O’Neill said. “And there are still trusted insiders. There’s still insiders because you’re never going to discard what’s worked for generations and generations. It’s just easier to steal someone’s credentials now than to recruit a source.”