The evidence is strong, but not yet sure, that Alexander Litvinenko, an exiled former KGB agent and vocal critic of Russia's Putin regime, was killed by a dose of a rare radioactive substance called polonium 210.
Both the KGB (now the FSB) and the CIA have records of developing toxic materials to use against their adversaries. I myself got a taste of KGB chemistry in 1956, when I flew to Yalta to cover a meeting between Nikita Khruschev and President Tito of Yugoslavia. The Kremlin didn't want the meeting to be covered. After breakfast in a seaside hotel, I began to feel funny and blacked out. When I regained consciousness, I was on an Aeroflot plane arriving in Moscow. The US embassy physician told me I had been slipped a Russian version of a Mickey Finn.
Other targets of the KGB or FSB didn't get off as easily. In 1978, a Bulgarian dissident was killed by a poison pellet fired from an umbrella on a London street. In 2004, Ukranian reformer Viktor Yushchenko was poisoned with dioxin, which left his face disfigured. He went on to win the Ukranian presidential election. In 2002, a Chechen rebel leader named Khattab died after opening a poison-soaked letter.
American spook-dom has its own annals of chemistry in the service of the cold war. On the very day that President Kennedy was assassinated in Dallas in 1963, a Cuban dissident met a CIA agent in Paris to receive a poison-tipped ballpoint pen, to be used against Fidel Castro. The agency also devised a diving suit dusted inside with a lethal powder. There is no sign that Castro ever saw the diving suit.
It may take a long time to solve the mystery of Litvinenko's death. Meanwhile, what fuels suspicion is that he was fighting to denounce Putin's regime for the death of crusading journalist Anna Politkovskaya. Proven or not, the radioactive death of Litvinenko hangs like a cloud over Vladimir Putin's head.
• Daniel Schorr is the senior news analyst at National Public Radio.