(CNN) Most US spy hunters work their entire careers tracking down undercover agents without ever personally playing a direct role in the physical arrest of a spy.

Robert David Booth, however, is the rare exception.

Why wiretapping is still used by agencies

Why wiretapping is still used by agencies 01:31

Why wiretapping is still used by agencies

The retired State Department deputy director of counterintelligence was able to investigate a spy and then actually help to plan and execute an arrest -- a rare thrill that Booth considers himself lucky to be a part of.

In early 1999, Booth and his colleagues learned that Russian spies were listening to classified conversations deep in the heart of Washington's State Department building -- inside a conference room in the same corridor as the Secretary of State's personal office.

The resulting investigation was code-named Sacred Ibis. Booth, also the author of "State Department Counterintelligence: Leaks, Spies and Lies," spent some time with CNN to talk about that investigation. Below is a transcript of our conversation, which has been edited for length and clarity.

The Russians are listening

ROBERT DAVID BOOTH: Sacred Ibis was brought to us by an FBI agent. Under my agreement [with the government] I can't use his name, but he showed me videotapes and still pictures of a Russian diplomat [named Stanislav Borisovich Gusev]. The intelligence community believed Gusev was a KGB technical officer working under diplomatic cover in the United States.

In one film, Gusev was sitting on a bench in the back of the main State Department building. He had (a) bag next to him, one hand in the bag, and an earphone plug was in his ear with the wire snaking into the bag.

Once we watched him sitting on the bench with one hand in the bag, one hand in his shirt pocket, smoking a cigarette all the while reading the Washington Post upside down. Days later, another film revealed him driving a car up the same street where the bench was located. He would park it, leave it for a couple of hours, then come back and drive off.

In reviewing the film, the intelligence community concluded that Gusev's actions were consistent with a technical operation. They believed that a bug — a listening device — had been placed inside the State Department's rear facade.

JUST WATCHED Russian diplomats entered the State Department without an escort Replay More Videos ... MUST WATCH Russian diplomats entered the State Department without an escort 00:52

We monitored the area, both physically and technically, and sure enough one of our countermeasure devices strongly indicated there was a listening device on the seventh floor. We surmised Gusev's car, equipped with Russian Embassy diplomatic plates, contained electronic equipment that would send a signal across the street, activating a bug as a live microphone that picked up conversations in a conference room.

One night we went into the State Department building at 2 o'clock in the morning and used additional equipment to positively identify the listening device that had been concealed inside a chair rail molding on the wall of a 7th floor conference room.

That was an a-ha moment.

To our incredible surprise, we learned that one group known as the D Committee met in this room to review the official personnel files of senior State Department people.

The committee discussed not only their active personnel file, but also talked candidly about their "corridor reputation." If the Russians were listening to those conversations, they would have learned about certain vulnerabilities that a number of senior State Department people had.

OK, Russia bugged us. Now what?

BOOTH: There was a big debate within the intelligence community about what to do about the listening device. One segment of the intelligence community said, "Let's leave the bug in place and see if someone tries to service it so we can arrest them on the spot." Others said, "We've gotta rip it out right away so we can analyze it and develop counter measures; maybe the Russians have similar devices in other areas, like US embassies or private businesses inside the United States."

Others said, "Why don't we just have people go into the conference room and have a controlled conversation that would give the Russians conniptions or give them false information?"

JUST WATCHED A listening device was planted inside the State Department Replay More Videos ... MUST WATCH A listening device was planted inside the State Department 01:10

What the intelligence community decided to do was monitor the conference room with microphones and video cameras to see if the Russians were "stealing" enough classified information to make an arrest for violations of the espionage laws and, maybe, to better understand why the Russians chose this conference room.

After about 30-35 days, the 24/7 monitoring operation had drained my office's resources. The decision was finally made that it was time to arrest the Russian diplomat, grab his car and take the bug off the wall so the intelligence community could analyze it.

But the day we were supposed to make the arrest, Gusev didn't show up.

Then he didn't show up for the rest of the week.

'An incredible moment'

BOOTH: On Monday morning I was in my office and some people were in a foul mood, thinking Gusev escaped arrest.

I had a couple of other cases I had to talk about with a diplomatic security special agent from my office who was monitoring this on the FBI side. I called him up on a secure, encrypted phone and all of a sudden he interrupts, "Robert, he's off the park."

That was a code word that Gusev had left the Russian embassy compound.

He was coming back down to park the car to [activate] the listening device.

That was one of those moments. I thought, "Oh God. We're gonna do it."

It felt great. All this work, the intelligence community is getting everything they wanted. We're going to arrest a Russian diplomat.

I'm sitting in my office window listening to FBI radio reports describing Gusev approaching the State Department in his diplomatic plated Chevy Malibu.

Then, from the sixth floor, I watched Gusev park his Malibu literally right below me in the street right at the entrance of my office. He had no idea it was Diplomatic Security's main office headquarters at the time.

Watching from my window, I'm having a heart attack. I'm going crazy. My poor secretary Teresa Black says, "Robert, you've gotta calm down" as Gusev started putting quarters into the parking meter.

And at that moment I said, "That's it. We've got him. We're gonna make the arrest. We're gonna get the bug. We're gonna get the car."

It was just an incredible moment.

CNN: Do you think something like this could happen again?

BOOTH: For a Russian or another hostile country's diplomat to get inside the State Department now and do the same thing — that's probably not going to happen.