“Preemptive assassinations, that was his thing. He wanted to create a deniable assassination squad,” admits David Cartwright – an old intelligence chief – in the latest season of the hit Apple TV series Slow Horses. Cartwright is talking about a character called Frank Harkness, a former CIA and US special forces agent gone rogue – the main enemy of Jackson Lamb’s singular intelligence outfit, the Slow Horses.

As the plot unfolds, we learn that Harkness’s project was to groom assassins from birth, separating them from their families and subjecting them to a programme of psychological conditioning and torture. “You start the training young,” Harkness reveals in the tense season finale: “You have a near invincible squad.” The trained assassin in the series, despite possessing almost superhuman strength, is terrified of Harkness and obeys his every command. He is the perfect assassin.

The notion of a perfect assassin has been a pervasive theme in pop culture, from books to TV shows and hit movies. But as is often the case, pop culture builds on historical foundations. The idea you could build a killer through hypnosis and psychological conditioning – or “brainwashing” – was a theme of early cold war experiments conducted by the CIA. As is often the case, facts and fiction have blended to inspire countless conspiracy theories.

In 1953, Colonel Frank Schwable, a US serviceman taken prisoner in the Korean war confessed under duress that he had taken part in the deployment of bacteriological weapons against China and North Korea – a confession later retracted. In the US, the episode created the fear that the Soviet Union and China possessed the capabilities to brainwash people, to get them to do or admit things they wouldn’t normally do.

In 1959, Richard Condon’s political thriller The Manchurian Candidate brought this fear to the masses. In Condon’s book, later a blockbuster movie, Sergeant Raymond Shaw is captured by a Soviet commando unit during the Korean war. Brainwashed by Communist officials, he later returns to the US as a sleeper agent with a sinister purpose. The plan unravels when one of Shaw’s friends and comrades discovers the hypnotic trigger – the card game solitaire – and uses it to “reprogramme” him, and prevent the assassination of a presidential candidate.

Similar tropes have appeared over the years. In the first season of Homeland, CIA operations officer Carrie Mathison becomes obsessed with Nicholas Brody, a US marine who’d been held captive by al-Qaeda. Mathison’s concerns – and much of the narrative – surround the risk that Brody has been turned and will conduct a terrorist attack in the US.

In Robert Ludlum’s Bourne series of novels (also made into movies) the CIA has groomed an elite group of assassins. When one fails a mission, he is dropped by the agency and pursued mercilessly. Jason Bourne cannot recall his past life, which only appears in quick flashes. Even when he claims to “remember everything”, he is told that remembering everything does not mean knowing everything.

The Treadstone TV series is a more recent spin-off of the Bourne universe. Operation Treadstone, the CIA programme that created Bourne, has also created scores of other sleeper agents through – we are told – behavioural modification protocols. These sleeper agents are ready to be reactivated and go on a killing spree. But did the CIA have an Operation Treadstone or an equivalent? The answer is yes and no.

Bluebird, Artichoke and MKUltra

In the early cold war, the CIA conducted many experiments in hypnosis, behavioural modification and brainwashing. The aim was “mind control”.

Hypnosis experiments started under project Bluebird. Project Artichoke followed in 1951. One of its aims was “the development of means for the control of the activities and mental capacities of individuals whether willing or not”. As part of Artichoke, the CIA conducted both official and unofficial experiments.

Morse Allen – the leading CIA official behind “behavioural research” – worked with hypnotists and conducted experiments on his own secretaries. In one case he successfully convinced one secretary to pull the trigger of an unloaded gun against another.

But the programme also ran on a more official footing. An Artichoke team was asked to work on a “hypothetical problem”, entailing whether a person could be programmed to assassinate the official of a foreign government or a US official. To this, the document added the disclaimer “simulated only”.

Having found a potential assassin and having struggled to control him, the Artichoke team concluded that the answer to the hypothetical problem was most likely no. A person could not be reprogrammed to assassinate even if – as Morse Allen had been told – the individual was a dubious moral character to start with.

And yet, Artichoke experiments continued and later expanded to experiments with drugs, electroconvulsive therapy and sensory deprivation, often in medical facilities through the infamous MKUltra programme.

The long shadow of ‘mind control’

MKUltra was shut down in the 1970s and most of the documents were destroyed. By then, the programme had developed a life of its own. In part, the families of MKUltra victims called for justice and challenged the US government’s secrecy.

Frank Olson, for example, was a scientist and employee of the US Biological Warfare laboratory. He was drugged unwittingly with LSD by the chief of MKUltra, Sydney Gottlieb, and a few days later fell to his death out of the window of a New York hotel in mysterious circumstances. His family received an apology from the Ford Administration when MKUltra was partially disclosed in the 1970s.

Similarly, families of Canadian patients who suffered in facilities and under programmes linked to MKUltra, in Montreal have been searching for justice since the 1980s.

In part, the programme lived on in conspiracy theories – some more credible than others – pointing to connections with MKUltra and techniques developed within it to explain the Manson murders and the assassination of Robert Kennedy. Finally, revelations surrounding MKUltra launched a new wave of pop-cultural products, including the 1970s conspiracy-ridden movie The Parallax View – which continued the tradition of the Manchurian candidate and, no doubt, inspired Jason Bourne, Frank Harkness, and the other perfect assassins to come.