The Russians have long used assassins to silence perceived enemies. One of the most celebrated at SVR headquarters in Moscow is Colonel Grigory Mairanovsky, a biochemist who experimented with lethal poisons, according to a former intelligence official.

Putin, a former KGB officer, has made no secret of his deep disdain for defectors among the intelligence ranks, particularly those who aid the West. The poisoning of Skripal at the hands of Russian operatives in Salisbury, Britain, in 2018 signalled an escalation in Moscow’s tactics and intensified fears that it would not hesitate to do the same on American shores.

The attack, which used a nerve agent to sicken Skripal and his daughter, prompted a wave of diplomatic expulsions across the world as Britain marshalled the support of its allies in a bid to issue a robust response.

The incident set off alarm bells inside the CIA, where officials worried that former spies who had relocated to the United States, like Poteyev, would soon be targets.

Putin had long vowed to punish Poteyev. But before he could be arrested, Poteyev fled to the United States, where the CIA resettled him under a highly secretive program meant to protect former spies. In 2011, a Moscow court sentenced him in absentia to decades in prison.

Poteyev had seemed to vanish, but at one point, Russian intelligence sent operatives to the United States to find him, though its intentions remained unclear. In 2016, the Russian news media reported that he was dead, which some intelligence experts believed might be a ploy to flush him out. Indeed, Poteyev was very much alive, living in the Miami area.

That year, he obtained a fishing license and registered as a Republican so he could vote, all under his real name, according to state records. In 2018, a news outlet reported Poteyev’s whereabouts.

The CIA’s concerns were not unwarranted. In 2019, the Russians undertook an elaborate operation to find Poteyev, forcing a scientist from Oaxaca, Mexico, to help.

The scientist, Hector Alejandro Cabrera Fuentes, was an unlikely spy. He studied microbiology in Kazan, Russia, and later earned a doctorate in the subject from the University of Giessen in Germany. He was a source of pride for his family, with a history of charitable work and no criminal past.

But the Russians used Fuentes’ partner as leverage. He had two wives: a Russian living in Germany and another in Mexico. In 2019, the Russian wife and her two daughters were not allowed to leave Russia as they tried to return to Germany, court documents say.

That May, when Fuentes travelled to visit them, a Russian official contacted him and asked to see him in Moscow. At one meeting, the official reminded Fuentes that his family was stuck in Russia and that maybe, according to court documents, “we can help each other”.

A few months later, the Russian official asked Fuentes to secure a condo just north of Miami Beach, where Poteyev lived. Instructed not to rent the apartment in his name, Fuentes gave an associate $20,000 to do so.

In February 2020, Fuentes travelled to Moscow, where he again met with the Russian official, who provided a description of Poteyev’s vehicle. Fuentes, the Russian said, should find the car, obtain its licence plate number and take note of its physical location. He advised Fuentes to refrain from taking pictures, presumably to eliminate any incriminating evidence.

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But Fuentes botched the operation. Driving into the complex, he tried to bypass its entry gate by tailgating another vehicle, attracting the attention of security. When he was questioned, his wife walked away to photograph Poteyev’s licence plate.

Fuentes and his wife were told to leave, but security cameras captured the incident. Two days later, he tried to fly to Mexico, but US Customs and Border Protection officers stopped him and searched his phone, discovering the picture of Poteyev’s vehicle.

After he was arrested, Fuentes provided details of the plan to American investigators. He believed the Russian official he had been meeting worked for the FSB, Russia’s internal security service. But covert operations overseas are usually run by the SVR, which succeeded the KGB, or the GRU, Russia’s military intelligence agency.

One of the former officials said Fuentes, unaware of the target’s significance, was merely gathering information for the Russians to use later.

Fuentes’ lawyer, Ronald Gainor, declined to comment.

The plot, along with other Russian activities, elicited a harsh response from the US government. In April 2021, the United States imposed sanctions and expelled 10 Russian diplomats, including the chief of station for the SVR, who was based in Washington and had two years left on his tour, two former US officials said.

Throwing out the chief of station can be incredibly disruptive to intelligence operations, and agency officials suspected that Russia was likely to seek reprisal on its American counterpart in Moscow, who had only weeks left in that role, the officials said.

“We cannot allow a foreign power to interfere in our democratic process with impunity,” President Joe Biden said at the White House in announcing the penalties. He made no mention of the plot involving Fuentes.