Aldrich Ames

Probably the most damaging turncoat in CIA history. A career agency official, Ames began selling U.S. secrets to the KGB in 1985, when he was head of the CIA's Soviet counterintelligence unit. Within a decade he had revealed more than 100 covert operations and betrayed at least 30 agents. Ten of the spies revealed by Ames were later executed by the Soviets, including Dmitri Polyakov, the top CIA informer inside Soviet military intelligence (GRU). Ames ' activities also may have allowed the Soviets to dupe the CIA by sending fake intelligence to the agency through the agents whom Ames compromised.