At the end of the Cold War, three Americans were caught for spying for the KGB. It is widely believed there was another. Robert Baer chronicles this on-going hunt in his book "The Fourth Man." In his visit on Maine Street, Baer said the FBI is closing in on the culprit.
http://accesswdun.com/article/2022/6/1108289/listen-the-hunt-for-a-kgb-spy-at-the-top-of-the-cia