The Russians, according to former officials, have used a variety of means to track the former informants.
Many, including the one in Florida, were relocated to the United States along with their family members, and Russians have tracked relatives’ social media accounts to find the families, according to 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. Alexander Zaporozhsky, a Russian colonel, defected to the United States 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 ofa spy swapwith the United States 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 C.I.A. officials said.
In late 2013 or early 2014, the Russian operative who traveled to Florida entered the United States on a valid visa, and American intelligence agencies, which knew enough about his identity to be concerned that he had traveled to the country, began tracking him and discussed whether to stop and question him.
But detaining a Russian who arrived in the country legally is difficult, F.B.I. officials said, and would be likely to prompt the officer to abort his operation, denying American counterintelligence agents a chance to gain valuable intelligence about his activities. Instead, officials decided to monitor the man and watched as he visited the Florida home of the former informant.
The decision to spy on the Russian was also a gamble and showed the level of concern by intelligence agencies. Surveillance is risky because trained spies can detect it. If spies determine how American intelligence officers spotted them, C.I.A. officials worry, that can compromise the method of spying.