WASHINGTON • When a suspected hitman for Russian intelligence arrived in Florida about four years ago, it sparked alarm among Federal Bureau of Investigation (FBI) surveillance teams.
The man approached the home of one of the Central Intelligence Agency's (CIA) most important informants, a fellow Russian, who had been secretly resettled there.
The suspected hitman also travelled to another US city where one of the informant's relatives lived, raising even more concerns that the Kremlin had authorised revenge on American soil.
At the FBI headquarters, some agents voiced concern that Russian President Vladimir Putin, himself a former intelligence officer known to reserve scorn for defectors from their ranks, had sent an assassin to kill one he viewed as a turncoat.
Ultimately, the Russian defector and his family remained safe.
But after the poisoning in March of Mr Sergei Skripal, a former Russian intelligence officer living in Britain, and his daughter, US intelligence officials have begun to reassess the danger facing former spies living in the United States, according to current and former intelligence officials who spoke on the condition of anonymity to discuss classified operations.
Moscow's intelligence agencies have in recent years tracked down several Russians who secretly served as CIA informants and were resettled in the US through a highly secret programme to protect former spies, according to the sources.
Counterintelligence officials have done a wide-reaching review of every former Russian informant now in the US, according to a US official. They have examined security measures to protect the former spies and searched for potential liabilities.
Both the former CIA informant in Florida and at least one other former CIA asset were resettled through the agency's protection division, the National Resettlement Operations Centre, after Russian intelligence found their homes, according to current and former officials.
Moscow's pursuit of informants intensified around the time relations with the West soured over Russia's 2014 annexation of Crimea. As Mr Putin has sought to reassert Russia's power, its largest military intelligence unit, the Main Directorate - also known as the GRU - has been linked to a number of brazen plots abroad, including the shooting down of a passenger jet over Ukraine and the theft of Democratic e-mail messages that was a major part of Russia's interference campaign in the 2016 US presidential election.
US officials have long believed that Mr Putin - who was sent home from his post as a KGB officer in East Germany during the fall of the Soviet Union - has a deep antipathy towards former intelligence agents who aid Western countries, but that he would be unwilling to order a strike in the US.
However, after Mr Skripal's poisoning, US intelligence agencies can no longer discount it.
Killing or even attempting to kill a former Russian spy in the US would not only further damage relations between Moscow and Washington but would also be likely to prompt an American response.
After Britain publicly accused Russia of poisoning Mr Skripal and his daughter with a military-grade nerve agent, the US, Britain and other Western countries expelled scores of Russian diplomats, plunging relations between the two sides into an even deeper freeze.
The Russian government already uses threats against former spies to try to intimidate current informants into going quiet and to dissuade others from aiding Western intelligence, current and former officials said.
In the mid-1990s, a former senior agency official said, the CIA located an explosive device under a car that belonged to a Russian intelligence officer who had spied for the CIA. At the time, it was not clear to the CIA whether the Russians intended for the device to explode or merely to serve as a chilling warning.
"The threat from Putin in this area is real and pervasive," said Mr Mike Rochford, a former FBI counterintelligence agent who helped expose Russian spies. "It is a dark legacy of a dead Soviet regime."
The Russians, according to former officials, have used a variety of means to track the informants.
Many, including the one in Florida, were relocated to the US along with their family members, and Russians have tracked their relatives' social media accounts to find the families, said former officials.
The Russians have also used more time-tested techniques, waiting for informants to grow homesick or using honey traps - fake romantic overtures - to lure a target.
Mr Alexander Zaporozhsky, a Russian colonel, defected to the US and lived quietly in Maryland until he decided to return to Russia in the early 2000s; a romantic interest lured him back, former agency officials said. He was taken into custody but freed as part of a spy swop with the US in 2010.
Defectors often reach out to friends and family in their native lands, communications that are typically vulnerable to eavesdropping by Russian intelligence officers, former CIA officials said.
Similar to the witness protection programme, the CIA's resettlement centre is responsible for more than 100 agency assets at any time. They are given US citizenship and asked where they want to live.
Many of them, a former senior CIA officer said, prefer Florida.
On a case-by-case basis, the CIA decides whether to give defectors new identities. In the case of Russians, the decision depends on how much Moscow knows about an informant and the level of interest from Russian intelligence.
But giving people new identities and hiding them in the US is becoming more difficult, according to former officials, in part because of the online presence of family members.
Many Russians and their families have been resettled over the years, including the intelligence officer who provided critical information about Mr Robert Hanssen, the former FBI agent who was convicted of espionage and sentenced to life imprisonment in 2002.
The CIA moved him and his son to a beach house in California nearly two decades ago, bought them BMWs and provided him an annuity worth millions of dollars.