These ex-spies apply a higher level of expertise, honed by government service, to the cruder tactics already practiced by private investigators. One such ploy is pretexting—obtaining information by pretending to be somebody else. While private detectives have long posed as freelance reporters or job recruiters to get people to talk, former agents have elevated pretexting to an art.

At Diligence, a New York private-investigation firm founded by former C.I.A. and British agents, ex-intelligence officers have taught newcomers how to construct false identities by using fake business cards, creating phony websites, and directing incoming calls to cell phones reserved for each separate identity. "You are establishing a cover, like in the C.I.A.," said a former Diligence employee, adding that there are people who know investigators only by their phony identities.

Similarly, ex-agents have helped popularize the use of G.P.S.-based monitoring devices and long-range cameras for following people around. One corporate-espionage technique comes straight from the C.I.A. playbook. In the constant search for the slightest edge, some hedge funds and investment companies have turned to a handful of private-investigation firms for a tactic that seems to fall between science and voodoo. Called tactical behavior assessment, it relies on dozens of verbal and nonverbal cues to determine whether someone is lying. Signs of potential deception include meandering off topic rather than sticking to the facts and excessive personal grooming, such as nervously picking lint off a jacket. This method was developed by former lie-detector experts from the C.I.A.'s Office of Security, which administers polygraph tests to keep agents honest and verify the stories of would-be defectors.

Don Carlson is the former chief executive of a Boston research-and-analysis firm, Business Intelligence Advisors, where ex-C.I.A. agents have turned the human-lie-detector technique into a business tool. Carlson said hedge fund managers have hired ex-C.I.A. polygraphers from B.I.A. to sit beside them as a company executive delivered a rosy business forecast. The former agents were supposed to signal the manager if they sensed that the executive was dissembling. Carlson said he is convinced that human lie detectors work, though others scoff at the notion.

B.I.A. did not return calls. But I was told that Cascade Investment, the vehicle set up by Microsoft founder Bill Gates to handle his wealth, was among the B.I.A. clients resorting to the human lie detector. Gates relied on B.I.A. investigators to analyze security risks in foreign countries that he and his wife, Melinda, plan to visit. Gates also employs a former C.I.A. agent as head of his personal security team.