Another possibility, Carlin said, is that intelligence services are trying to corruptly profit from insider information they gather on the Street. "They want to make some money off what they learn from the stock exchange, just like regular crooks," he said.
Carlin said his message to Wall Street about foreign spies is simple: "They want what you have."
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Last week, federal officials brought charges against three men they said were undercover officers of the Russian intelligence service known as the SVR, a successor to the Soviet KGB. Two of the men had diplomatic immunity and were able to leave the country. But the third, Evgeny Buryakov, 39, was arrested in the Bronx.
The Russian media outlet Tass quoted a Russian consular official as saying that Buryakov "vehemently denies the alleged offenses."
The DOJ's Carlin declined to name other foreign intelligence services operating on Wall Street. But although today's spies use all the telecommunications and information-gathering technologies of the 21st Century, Carlin said they're still using age-old human weaknesses to find people who might cooperate—for any reason.
"Since the dawn of time people have been motivated by traditional human motives," Carlin said. "Revenge—they're angry at something that happened at the company. Sex—they're persuaded to give a secret because a young, attractive woman comes and tries to get information out of them. And greed—they're paid off for the secret."
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But finding those weaknesses now depends on some of that new technology, including surveillance of social media posts by people on Wall Street.
CEOs, Carlin said, now have to worry about what their employees are revealing about their innermost selves on social media: "We know spies and others are looking at what your employees are posting online to learn about them, to figure out who is most vulnerable so they can approach them and steal your secrets," Carlin said.
At the same time, he said it would be a mistake for a firm to simply focus on employees from one ethnic group or religious background. Instead, he said, CEOs should look for other potential risk factors.
"Unexpected behavior inside of your system. Anomalous behavior. Someone who has unexpected wealth that you haven't seen before," Carlin said. "And of course, if you have someone, a disgruntled employee, that can be a warning sign that they want to get back at the company."
Carlin said he wants companies to cooperate with the FBI—and contact their local U.S. attorney's office if they have specific concerns.