Washington (CNN) Years after defecting from Russia and resettling in the US, former KGB officer Alexander Zaporozhsky was lured back to the country he had once betrayed. The CIA warned him not to go.

Several senior intelligence officials tried unsuccessfully to dissuade Zaporozhsky, who had given the US valuable information that ultimately led to the capture of FBI agent and Russian spy Robert Hanssen, former officials familiar with the case told CNN.

But the Russian intelligence officer turned US asset insisted on returning to Moscow in 2001. He was arrested and imprisoned in Siberia for his connections to the West.

Zaporozhsky was ultimately released in 2010 and sent back to the US as part of a spy swap with Russia that also included Sergei Skripal, a former KGB colonel convicted of spying for Britain after admitting to the crime. The US released 10 individuals accused of operating a Russian spy ring in the US as part of the exchange.

The former KGB officer has largely flown under the radar since his release and his whereabouts remain a secret, but Skripal's name publicly resurfaced in March when he was poisoned with a nerve agent in the English city of Salisbury -- an incident that the UK and US allege was the work of the Russian government. Russia denies any involvement.

'Crown jewels of human intelligence collection'

Human intelligence collection and the CIA's recruitment of foreign spies was crucial to US efforts in the Cold War and continues to play a major role in the evolving world of global espionage.

On one hand, intelligence agencies like the CIA are constantly working to identify and apprehend foreign agents operating within the US on behalf of aggressive adversaries like Russia, China and Iran -- a concern highlighted by the ongoing criminal prosecution of 29-year-old Maria Butina, an alleged Russian operative.

Butina has pleaded not guilty to charges of conspiracy and acting as a foreign agent in the US. Her lawyer, Robert Driscoll, told CNN Wednesday that she wouldn't take a deal from prosecutors if it meant admitting she was a spy.

But the flipside is a shadow world where defectors like Zaporozhsky and Skripal are the ultimate trophy -- and take the ultimate risk.

In the US, a CIA program focuses on secretly rescuing and resettling spies like them who are in danger of being caught or killed for betraying their country in service to Washington.

The risks don't stop once they've reached US shores, former intelligence officials told CNN, as their former countries often continue to hunt them down. Those challenges have only gotten tougher as cyber capabilities advance and the evolution of social media make the task of protecting defectors even more difficult.

"For decades, high-level defectors have been the crown jewels of human intelligence collection," according to Joe Augustyn, a retired CIA officer who ran the defectors program for three years.

"Given the technology available to our adversaries and difficulty of hiding in plain sight -- the challenge the CIA has of protecting the spies among us has never been greater," Augustyn said. "Protecting these individuals who have betrayed their countries to help ours is even more important and challenging today than it was during the Cold War."

Photos: Spying on their own countries Spying on their own countries – Several American turncoats have been caught spying on their own countries while working at the CIA, the National Security Agency, the US State Department and the Pentagon. Here's a quick look at who got caught and how they were punished. Hide Caption 1 of 10 Photos: Spying on their own countries Harold James Nicholson – After what looked like a successful 16 years in the CIA, Harold James "Jim" Nicholson was caught selling secrets to Russia. He was convicted on espionage charges in 1997 and sentenced to 23 years in prison. In 2011, Nicholson's son Nathaniel was charged after meeting with Russian agents to collect money owed to his father. He was sentenced to five years probation. As a result, Harold Nicholson was sentenced to an additional eight years in prison on charges of conspiracy to act as an agent of a foreign government and conspiracy to commit money laundering. Hide Caption 2 of 10 Photos: Spying on their own countries Walter Kendall Myers – Retired US State Department analyst Walter Kendall Myers, left, pleaded guilty to wire fraud and conspiracy to commit espionage for Cuba in 2009. His wife, Gwendolyn Myers, right, pleaded guilty to conspiracy to gather and transmit national defense information. In 2010 Myers was sentenced to life in prison. His wife was sentenced to 81 months. Hide Caption 3 of 10 Photos: Spying on their own countries Ana Montes – American citizen Ana Montes worked for the Pentagon intelligence arm, the Defense Intelligence Agency, for 16 years before she was caught spying for Cuba. In 2002, she pleaded guilty to espionage and was sentenced to 25 years in federal prison. Hide Caption 4 of 10 Photos: Spying on their own countries Aldrich Ames – Aldrich Ames, a 31-year CIA employee, pleaded guilty to espionage charges in 1994 and was sentenced to life in prison. Ames was a CIA case worker who specialized in Soviet intelligence services and had been passing classified information to the KGB since 1985. US intelligence officials believe that information passed along by Ames led to the arrest and execution of Russian officials they had recruited to spy for them. Hide Caption 5 of 10 Photos: Spying on their own countries Jonathan Pollard – Jonathan Pollard is a divisive figure in US-Israeli relations. The former US Navy intelligence analyst was caught spying for Israel in 1985 and was sentenced in 1987 to life imprisonment. Previously, the United States and Israel discussed his possible release as part of efforts to save fragile Middle East peace negotiations, according to sources familiar with the talks. In 2015, Pollard was released on parole -- 30 years after his arrest. Hide Caption 6 of 10 Photos: Spying on their own countries Robert Hanssen – Robert Hanssen pleaded guilty to espionage charges in 2001 in return for the government not seeking the death penalty. Hanssen began spying for the Soviet Union in 1979, three years after going to work for the FBI, and prosecutors said he collected $1.4 million for the information he turned over to the Cold War enemy. In 1981, Hanssen's wife caught him with classified documents and convinced him to stop spying, but he started passing secrets to the Soviets again four years later. In 1991, he broke off relations with the KGB, but resumed his espionage career in 1999, this time with the Russian Intelligence Service. He was arrested after making a drop in a Virginia park in 2001. Hide Caption 7 of 10 Photos: Spying on their own countries John Walker – John Walker ran a father and son spy ring, passing classified material to the Soviet Union from 1967 to 1985. He was a Navy communications specialist with financial difficulties when he walked into the Soviet Embassy and sold a piece of cyphering equipment. Navy and Defense officials said that Walker enabled the Soviet Union to unscramble military communications and pinpoint the location of US submarines at all times. As part of his plea deal, prosecutors promised leniency for Walker's son Michael Walker, a former Navy seaman. Michael Walker was released in 2000 after serving 15 years in prison. John Walker died in prison in 2014 Hide Caption 8 of 10 Photos: Spying on their own countries Ronald Pelton – Ronald Pelton joined the super-secret National Security Agency, the US government's electronic intelligence arm, after serving in the Air Force's communications intelligence division. He resigned in 1979, but after running into financial trouble, he approached the Soviet Embassy in Vienna. He then began passing Moscow classified information, including details of a program that tapped undersea Soviet communications cables. He was exposed when his handler defected to the United States in 1985. Although he was sentenced to three concurrent life sentences, Pelton eventually was released in 2015. Hide Caption 9 of 10 Photos: Spying on their own countries Earl Pitts – Earl Pitts' job was to monitor suspected Soviet spies at the United Nations. But the veteran FBI agent soon began selling his secrets to the KGB, which he contacted in 1987, and its successor agencies after the Soviet Union collapsed. He got about $224,000 from the Kremlin before a Russian double agent tipped off US intelligence, and he was arrested in 1996. The federal judge who sentenced him to 27 years in prison -- more than prosecutors had requested -- told him, "You betrayed your country, you betrayed your government, your fellow workers and all of us, really." Hide Caption 10 of 10

Former CIA director Gen. Michael Hayden told CNN that defectors don't just help with current or past intelligence.

"We have found that defectors are a long-term and deep source of information and that they enlighten things we may have already collected but not understood, as well as those things we might collect in the future," Hayden said.

While the CIA intentionally maintains a level of operational secrecy to protect defectors, events like Skripal's poisoning are a reminder that the stakes are as high as ever for those who participate in clandestine operations and the handlers responsible for their well-being.

"When I would speak at the graduation of CIA case officers, I would remind them of the moral responsibility they had to anyone they would recruit," Hayden said. "That individual was putting his or her welfare and that of their entire family into the case officer's hands. And the case officer might be the only face of America that source might ever see."

Hiding in plain sight

The CIA's defector program has existed since 1949 when Congress passed a law authorizing the agency to resettle up to 100 foreign nationals in the US if it's "in the interest of national security or essential to the furtherance of the national intelligence mission."

While the CIA doesn't take advantage of the full quota that is allowed each year, the reality is that there are hundreds of former spies living in the US with new identities -- resettled into communities across the country.

The dozens of CIA officials who work for the defector program are responsible for keeping track of those individuals and advising them as best they can for the rest of their lives -- all the while knowing that foreign adversaries continue to aggressively search for them.

Indications that Russia was behind Skripal's poisoning provides further evidence that Moscow remains particularly vigilant in their quest to find those who betray the motherland -- at times demonstrating a unique willingness to patiently wait for the opportunity to retaliate.

"As we have seen recently in the UK, the risk to these defectors does not end when they reach the west. It is critical that US intelligence does all that it can to make sure they live safe and productive lives not only as a 'thank you' for what they have done -- but also as a message to future defectors that the United States lives up to its commitments," Augustyn said.

Not every spy qualifies for exfiltration, but the US will work to rescue high-ranking military officials, intelligence officers, academics, scientists and other workers with access to sensitive information who are at imminent risk of being caught or captured.

If the defector is successfully brought out of their home country, often in the dead of night, he or she and their family will be given new names, English lessons, some money, a house, and words of advice: don't use social media, don't contact anyone back home.

In one case, the CIA helped a man who wanted to go to college by providing fake records he needed to apply, while in another, officers facilitated a secret divorce for an asset whose wife still lived in the country he defected from.

Psychiatrists work in the special unit as well, to help families adjust to their new surroundings.

Those guidelines aren't always followed.

Some of the children of former spies have been unable to resist social media, and there have been defectors who have returned to their home countries after failing to adjust to life in the US.

One former spy sued the agency for failing to provide his family with sufficient funds, and the case escalated all the way to the Supreme Court -- which found in the CIA's favor, thanks to a long-forgotten Civil War-era law concerning secret agreements called the Totten Doctrine that protected Abraham Lincoln from being sued by a spy he paid to follow Confederate soldiers.

Complex challenges

Defectors present a myriad of psychological and logistical challenges for CIA handlers as they navigate the complex reality of life-long resettlement.

Some defectors maintain an "unnaturally strong belief of their own invulnerability" that can manifest itself years into the program -- a trait that often makes them targets of coercion and can have potentially fatal consequences, according to Augustyn.

Other individuals cannot reconcile themselves to the fact that they are no longer the person they used to be, he added.

"Anyone who betrays their country, for whatever reason, is different," Augustyn told CNN, adding that these individuals often have strong egos and are motivated by personal reasons like greed or a desire for revenge rather than political ideology or a desire to make the world a better place.

And while the CIA provides defectors with new identities and professions, the jobs given are often a dramatic step-down from the high-level military, political or intelligence professions that made them attractive recruitment targets in the first place.

One high-level defector even became a pizza deliveryman upon resettling in the US, according to Augustyn.

That change in lifestyle can be jarring for defectors and their families who are forced to abandon parts of their life that long served as the foundation of their identity.

"Each case is unique," Peter Earnest, a 35-year CIA veteran who helped resettle multiple former Soviet officials in the US, told CNN. But "people who have served us in difficult circumstances, we owe them an obligation," he added.

Russia is not the only foreign adversary interested in tracking down defectors living in the US. The CIA has resettled individuals from countries like China, Iran and North Korea and those countries also want to track down those they believe have betrayed their interests.

While CIA officials would not say whether the US has noticed a recent increase in the targeting of defectors, intelligence officers constantly evaluate safety concerns of those spies secretly living in the US.

The CIA's defector program remains sensitive, and few people, including lawmakers and former intelligence officials, will discuss it, but those who will say the defectors program it helps the US extract valuable assets from dangerous situations, promoting loyalty and creating an incentive for anyone considering choosing to spy for the US in the future.

"Beyond the moral compulsion to do right by defectors, the nation that does not protect those who defect will find that fewer people will be making that choice," Hayden told CNN.

But while the CIA has successfully resettled hundreds of former spies over a span of several decades, many cases have ended in tragedy.

Since the program was founded after World War II, several defectors from Eastern Bloc countries alone have been assassinated or disappeared under suspicious circumstances.

The CIA is still searching for answers related to the case of Ryszard Kuklinksi, a Polish army colonel who gave the US thousands of documents revealing sensitive information about the Soviet nuclear weapons program -- he was exfiltrated from Poland by the CIA just before the Polish government instituted martial law in 1981.

Kuklinski lived peacefully in Florida for many years after leaving Poland and was celebrated as a hero within the US intelligence community for dramatically improving their understanding of Soviet weaponry in the 70s and 80s.

However, his two sons mysteriously died within six months of each other in 1994 -- one lost at sea off the coast of Florida and the second hit by a car, which burned up and left behind no evidence of the perpetrator. The mystery remains unsolved, though many have pointed the finger at angry KGB agents seeking retribution.