Washington (CNN) Somewhere in the halls of the J. Edgar Hoover building in Washington, a closely held number of FBI agents face the daunting task of determining how the Russian government sought to manipulate the US presidential election.

It's a case that's been the subject of endless speculation in Washington and beyond.

But to those tasked with getting to the bottom of the allegations and innuendo, it's just another "standard counterintelligence investigation," as one highly placed US official put it. Add to that the freshly assigned task of determining how a huge cache of what appear to be authentic CIA documents ended up on the WikiLeaks website.

Welcome to the super-secret world of the Counterintelligence Division, home to the spy catchers of the FBI.

It's a shadowy world that's closed off even to those typically in the know in bureau headquarters, a silo of secrecy in which agents are valued as much for their ability to keep quiet as they are for their investigative skills.

"They keep that (stuff) locked down tight," one veteran FBI agent said.

One source familiar with the Russia investigation resorted to a mathematical equation to divulge -- sort of -- the number of agents assigned to the matter.

It's five to 10 fewer than were assigned to the Hillary Clinton email investigation, said the source, who is not authorized to speak publicly and did so on the condition of anonymity. There were about two dozen dedicated to that case, so that makes 15 to 20 on the Russia investigation.

The resources assigned to the Clinton investigation were in response to agents having to sort through a vast amount of electronic data in a finite period of time before the then-looming presidential election, the source said. With the Russia probe, there is no such time pressure and efforts are more focused on interviews with human sources.

The smaller number of agents assigned to the case should not be interpreted as a lack of interest, the source said. Developments in the case are sent up the chain to the highest levels on a regular basis.

Known simply as CD within the bureau, the Counterintelligence Division is responsible for protecting the secrets of the US intelligence community, the advanced technologies of American institutions both public and private, keeping weapons of mass destruction away from US enemies and countering the activities of foreign spies, including cyberintrusions.

Their cases sometimes go on for years amid careful, tedious, behind-the-scenes work aimed at recruiting or neutralizing foreign spies.

One reason for the long, drawn-out investigations is that their cases rarely are as cut and dried as bank robbery or kidnapping.

In the Russia investigation, for instance, there's been abundant speculation about the significance of meetings between people connected to President Donald Trump and Russian Ambassador to the US Sergey Kislyak.

A meeting in itself means nothing, said Mike Rogers, a former FBI agent and Republican congressman from Michigan.

"First of all, they have to find a crime," Rogers, a CNN national security commentator, said of the agents.

"I have met with Kislyak twice, as a congressman," Rogers said. "That's not a crime."

Rarely do they result in criminal prosecutions. Rarer still are headlines.

There was an exception in 2015, when the Counterintelligence Division was publicly credited with the disruption of a Russian spy ring operating in New York City. It was lead by a man working in a Russian bank in Manhattan, according to federal authorities. The man, Evgeny Buryakov, also happened to be an agent of Russia's SVR, their equivalent of the CIA. It was a spy-novel-worthy case involving electronic intercepts, surveillance and coded messages. Buryakov pleaded guilty last year and was sentenced to two and half years in prison.

Ironically, the division's most enduring publicity came in the wake of the massive betrayal of one of its longtime agents: Robert Philip Hanssen.

Hanssen was arrested in 2001 and charged with selling secrets to the former Soviet Union and Russia over the course of two decades. He was paid hundreds of thousands of dollars in cash and diamonds in exchange for secrets that he often left at a "dead drop" location for his handlers to retrieve. He was arrested following what would be his final drop, pleaded guilty and is serving a life sentence in prison. He is considered the most damaging spy in FBI history.

But following his arrest, then-FBI Director Louis J. Freeh singled out Hanssen's colleagues in the Counterintelligence Division for confronting "the most traitorous actions imaginable."

"Their actions represent counterintelligence at its very best and under the most difficult and sensitive of circumstances," Freeh said. "Hanssen's colleagues and coworkers at the FBI conducted this investigation and did so quietly, securely and without hesitation. Much of what these men and women did remains undisclosed but their success and that of their CIA counterparts represents unparalleled expertise and dedication to both principle and mission."

CD is so secretive that even former agents are reluctant to speak publicly about their tenure there.

One former supervisor said agents need the seemingly disparate skill set of being good talkers who can recruit and maintain sources while also being able to keep their mouths shut -- about everything.

"Ninety-five percent of the cases are classified information or above," the source said.

"You have to be very disciplined about what you share with whom," he said. "My wife knew where I worked. She did not really know what I did."

He said counterintelligence work would not appeal to someone with the stereotypical macho cop persona or someone who wants to be the center of attention.

"It's more subtle. You're working in the shadows. You don't want to be noticed," the source said. "It's very different than kicking down doors or doing drug raids."