THE DISCLOSURE that a clandestine unit in Russia’s Federal Security Service, the FSB, used a chemical nerve agent in the assassination attempt against opposition leader Alexei Navalny is genuinely alarming. It appears that President Vladimir Putin has deployed the FSB to kill his leading critic and challenger. It also suggests Russia has blatantly violated the Chemical Weapons Convention, an international treaty it signed and ratified. President Trump’s response has been pitiful. The United States cannot fail to act.

In October, Bellingcat, the open-source investigative outfit, and its partners reported that Russia’s research institutes were continuing to work on the class of Cold War-era military nerve agents known as Novichok at facilities disguised as civilian research. The Chemical Weapons Convention prohibits the development, production, acquisition, stockpiling, retention, transfer or use of chemical weapons. Mr. Putin says Russia has strictly adhered to its commitments.

But the latest reports indicate otherwise. A new investigation published by Bellingcat, along with the Insider, a Russian news organization, Der Spiegel of Germany, and CNN, revealed a clandestine unit within the FSB that specializes in poisonous substances. The investigation, using telephone records and air travel manifests, documented years of surveillance of Mr. Navalny starting in 2017, shortly after he first announced his intention to run for president of Russia. Throughout 2017, and again in 2019 and 2020, FSB officers in this clandestine unit shadowed him on 37 overlapping flights to the same destinations in Russia. Mr. Navalny fell ill on a flight from Tomsk to Moscow on Aug. 20. The plane made an emergency landing in Omsk, where he was treated before being evacuated to Berlin.

Laboratories in three countries and the Organization for Prohibition of Chemical Weapons confirmed the use of Novichok agents against Mr. Navalny. The Bellingcat investigation identified the members of the FSB squad with names, photographs and details of the surveillance. “I know who wanted to kill me,” Mr. Navalny said in a video documenting the attempt. “I know where they live. I know where they work.”

The Bellingcat probe suggests the FSB, supposedly a law enforcement body, serves as Mr. Putin’s extrajudicial death squad. While the Chemical Weapons Convention has allowances for developing antidotes and defenses against chemical weapons, producing and using Novichok agents to poison Mr. Navalny are a brazen treaty violation. A U.S. response is essential. The House last month approved a resolution calling for tougher action, and a bipartisan group of senators has proposed sanctions. Yet when Mr. Trump, whose affinity for Mr. Putin has never been satisfactorily explained, was asked by a reporter on Sept. 21 who might have poisoned Mr. Navalny, he responded: “We’ll talk about that at another time.”

The administration has barely slapped Russia on the wrist, putting two Russian facilities suspected of chemical weapons development on a Commerce Department blacklist. We don’t have much hope Mr. Trump will do more. But President-elect Joe Biden should make it clear the United States will not excuse such dangerous behavior, and impose stricter sanctions on Russian officials, including those measures available under the Magnitsky Act and other laws.

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