Emergency workers in protective suits search around a homeless hostel that was home to one of the victims in the Novichok poisoning in Salisbury, England. (Jack Taylor/Getty Images)

WHEN TWO Russians carried radioactive polonium to London in 2006 in a murder plot against Alexander Litvinenko, a former officer of the Federal Security Service turned critic of President Vladi­mir Putin, they left behind a trail of the substance. They poisoned Mr. Litvinenko by putting it in his tea, and he later died, but they also left traces in hotels and a restaurant and aboard several airplanes, among other places. It was a brazen and ugly assassination in an open society, for which the perpetrators escaped punishment, and a British inquiry later concluded Mr. Putin “probably” approved the killing.

Now, another dangerous substance has been strewn around Britain, killing another person, and once again the urgent question is whether it came from Russia, as appears likely.

In early March, in the British town of Salisbury, a former Russian intelligence officer turned double agent, Sergei Skripal, and his daughter, Yulia, were discovered poisoned by Novichok, a binary nerve agent developed in the last years of the Soviet Union and believed to be 10 times more powerful than VX nerve gas. Fortunately, Mr. Skripal and his daughter recovered sufficiently to leave the hospital. No one ever identified who poisoned them, although Russia was highly suspect because Mr. Skripal had betrayed the intelligence services there, and Russia might still have samples of Novichok from Cold War times.

Then on June 30, two more Britons, Charlie Rowley and Dawn Sturgess, were sickened with Novichok in Amesbury, eight miles north of Salisbury. Ms. Sturgess died Sunday. The two appeared to have no ties to Russia, but Mr. Rowley was known as an avid “skip diver,” a scavenger of trash bins who searched for valuables that others may have thrown away. Perhaps he came across an attractive-looking thermos or something else that had carried the Novichok originally used against the Skripals. In both cases, Russia has denied any responsibility and crudely sought to deflect blame on others.

The Skripal poisonings triggered expulsions of Russian diplomats and intelligence officers from the United States and Britain, and reciprocal action from Moscow. But it cannot be left there. Those who weaponized Novichok must not be allowed to get away with this kind of banditry and murder on foreign soil, as Mr. Litvinenko’s killers did.

In June, a proposal to strengthen procedures for attribution in chemical weapons attacks — in other words, figuring out who did it — was approved by the nations that signed the Chemical Weapons Convention prohibiting the use of chemical weapons. The initiative was pushed by Britain, backed by the United States and approved by 82 nations, while opposed by 24 of those present. The opposition included Russia and Syria, where the Moscow-backed regime has regularly used chemical weapons in the civil war. If translated into effective action, this initiative could provide more teeth to investigations, helping detectives finger the perpetrators.