Alexander Litvinenko, who died last week of what may well have been polonium poisoning, was, like President Putin, a former KGB spy. But unlike Mr. Putin, Mr. Litvinenko saw the error of his ways, defecting to Britain in 2000 after leaving the FSB, the KGB’s successor agency, in a row with his superiors. Before his apparent poisoning, Litvinenko had been most famous in Russia for accusing FSB agents of participating in a 1999 apartment-building bombing plot that killed 300.

It’s not escaping notice that he appears to have been poisoned while pursuing an investigation of the recent assassination of Anna Politkovskaya, a reporter shot in her apartment building after years spent criticizing Mr. Putin’s government for its handling of the Chechen war. At least one of Litvinenko’s friends is claiming that the Russian government slipped the dangerous dose into the erstwhile spy’s food. The Kremlin denies any such allegation, but, as fantastical as it sounds, such behavior would certainly be in line with Moscow’s tendencies.

A police investigation is underway, but given the high profile of the victim, the bizarre circumstances, and the unlikelihood that such a plot would trace to anywhere other than the Kremlin, it’s worth considering forming an international commission to conduct its own investigation, such as the panel that has been probing the assassination of a Lebanese prime minister, Rafik Hariri.

Just as in the Lebanese case, in which Syrians have been suspected of orchestrating a murder in Beirut, the Litvinenko affair points up the potential for a heavy-handed dictatorship in one country to spill into another. Britain has a far more robust counterterrorism and criminal investigation program than does Lebanon, but if London is to take on Moscow, it may well need allies. NATO would be a better venue for this than the United Nations.

A political poisoning, if that is what happened, is more than a crime — it’s a national security problem for any country in which the next Litvinenko might find himself or herself living. Mr. Putin can’t escape the suspicion that his old habits haven’t died at all. The rest of the world can’t escape the possibility that those habits will affect free countries if left unchecked.