In the United States alone, one expert estimates that there are about 100,000 foreign agents working for at least 60 to 80 nations -- all spying on America.

"That's not paranoia -- that's a good guess," said Chris Simmons, a retired counterintelligence supervisor for the U.S. Defense Intelligence Agency, appearing on CNN's "Declassified."

It all sounds very much like the exciting Hollywood dramas set during the Cold War with Soviet spies pretending to be harmless citizens -- like F/X's popular TV series "The Americans."

Authorities warn that spies in America are not a thing of the past.

"More than two decades after the end of the Cold War, Russian spies still seek to operate in our midst under the cover of secrecy," U.S. Attorney Preet Bharara said in a statement last March.

Evgeny Buryakov, 41, posed as an employee in the Manhattan office of a Russian bank. He entered the United States and stayed as a private citizen, the Justice Department said.

Buryakov gathered "intelligence on the streets of New York City, trading coded messages with Russian spies who send the clandestinely collected information" to Russia's foreign intelligence agency, the SVR, Bharara said. According to court documents, Buryakov was "receiving taskings from Moscow."

He reportedly was sentenced to 2.5 years in prison and ordered to pay a $10,000 fine.

The FBI gathered some of the evidence in the case with basic techniques that might seem ripped from spy dramas.

In fact, despite all the technological advances since the Cold War, spy hunters still use and follow a lot of old-school espionage methods to catch spies. That includes stakeouts, covert breaking and entering, cryptography -- aka code breaking -- and dead drops.

NOCs: Non-official cover/secret identities

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U.S. authorities said Buryakov was spying undercover as a bank employee. In the spy game, that's known as working under non-official cover -- or NOC.

NOCs pose as members of other professions -- like business people, engineers or scientists. They have fake names, fake tax records and lie to their friends and family about their real identities. They also often create phony business fronts to get closer to their targets.

The life of a NOC is dangerous.

If they're discovered by foreign intelligence abroad they have no diplomatic immunity, putting them at risk of being arrested, thrown in prison or, in a worst-case scenario, executed.

Stakeout technology

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One of the most notorious stakeouts in FBI history was created in 1934 to catch Public Enemy No. 1: Gangster John Dillinger.

Agents got a tip that Dillinger, who'd been on the run from the FBI, was headed to one of two possible movie theaters in downtown Chicago. Two agents were posted at each movie house, and when Dillinger was spotted entering the Biograph Theater, the FBI office was called for backup.

Agents were posted at the Biograph's entrance, alley exits and across the street, with the intention of arresting Dillinger when he left the building.

When the movie was over, Dillinger came out of the theater with two women.

Agents started to move closer.

Dillinger "became apprehensive and started to run -- grabbing for his gun." Three agents open fired on Dillinger, who was pronounced dead at the hospital.

Obviously, with advanced technology, stakeouts have come a long way since those days.

With a covert listening device, someone who is conducting surveillance from a distance can capture audio around a specific target.

By using laser detectors pointed at a parked car, an observer can know when a target is using that vehicle to drive away. The target who is driving the car might be tracked via their mobile phone through geo-location.

Covert breaking and entering

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If they've done their job right, experts at covert breaking and entering will come into a home, search it -- and leave without a trace. The owners will never know that strangers have been there.

In the spy game, these searches are sometimes called "sneak and peeks."

When President Jimmy Carter signed the Foreign Intelligence Surveillance Act (FISA) into law in 1978, it set up a system for American spies to legally search and bug homes of people spying for the other side.

Spies have developed tactics that include:

photographing the entire home first and then using those photos later to make sure nothing has been moved out of place.

using fake dust to avoid leaving trails of movement.

In 1982, the FBI used FISA to bug the phones of Gabriel Megahey and Eamon Meehan -- members of the Irish Republican Army who were suspected of smuggling guns, explosives and surface-to-air missiles to the Provisional Irish Republican Army in Northern Ireland.

That FISA search led to their arrest and the arrest of two others. All four were found guilty.

Cryptography

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In a way, spies invent their own languages. That's cryptography -- encrypting or deciphering messages in secret code intended only for certain people to understand.

The Nazis used their famous Enigma machine to hide their military transmissions during World War II. A team of Allied code breakers cracked the Enigma code -- possibly shortening the war by at least two years.

Digital Symmetric-key cryptography has made reading secret messages infinitely harder. A message encrypted with a "key" is sent to someone with an identical "key" to decode it. The conversation is very secure. A 128-bit key can have more than 300 decillion combinations. That's a 3 followed by 35 zeroes.

Former FBI agent Robert Hanssen, one of the most infamous spies in U.S. history, was arrested while making what's known in the espionage trade as a dead drop.

A dead drop is a hiding spot spies use to secretly pass packages and information in a way that avoids them having to be in the same place at the same time.

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In 2001, Hanssen was arrested when he was caught leaving packages of secret U.S. intelligence at a dead drop site outside Washington in Vienna, Virginia.

The FBI said Hanssen had used dead drop sites multiple times to leave packages for his Russian handlers throughout the Washington area.

Inside those packages were discs and documents of valuable U.S. intelligence that compromised a wide range of FBI secrets, including counterintelligence investigative techniques, sources, methods and operations.

In exchange, Hanssen had been paid more than $600,000 in diamonds and cash, according to the FBI.