Espionage is a high-risk criminal offense. The traitor must fear arrest for the rest of his or her life, as the statute of limitations does not apply to espionage. Former National Security Agency employee Robert Lipka was arrested in 1996 -- 30 years after he left NSA and 22 years after his last contact with Soviet intelligence.
There are four principal ways by which spies are detected:
U.S. Sources Within the Foreign Intelligence Service: Of the Americans who held a security clearance who have been arrested for espionage, about half were caught as a result of information provided by a defector from the foreign intelligence service, or a penetration agent or friend within the foreign service that the spy was working for. People who betray their country often have little fear of being caught, because they think they are smarter than everyone else. They think they can easily get away with it. Ironically, even if they actually were smarter, it would not help them. No matter how smart or clever a spy may be, he or she has no protection against U.S. Government sources within the other intelligence service.
Changes in Behavior: Espionage usually requires keeping or preparing materials at home, traveling to signal sites or secret meetings at unusual times and places, change in ones financial status with no corresponding change in job income, and periods of high stress that affects behavior. All of these changes in normal pattern of behavior often come to the attention of other people and must be explained. Other people become suspicious and pass their suspicions on to others. This sometimes comes out during the periodic security clearance reinvestigation.
Spying is a lonely business. To explain these changes in behavior, or because of a need to confide in someone else, spies often confide in a spouse or try to enlist the help of a friend. The friend or spouse in whom the spy confides often does not remain a friend or loyal spouse after he or she realizes what is going on.
A person who helps stop espionage is eligible for a reward of up to $500,000. (See Counterintelligence Indicators for details.)
Irrational Behavior: Most people who betray their country are not thinking rationally, or they would not be involved in such a self-destructive activity. They are driven, in large part, by irrational emotional needs to feel important, successful, powerful or to get even or to take risks. These emotional needs are out of control, so the same emotional needs that lead them to betray also cause them to flaunt their sudden affluence or to brag about their involvement in some mysterious activity. Because they are so mixed up psychologically, they make mistakes that get them caught.