THE BIDEN administration has quickly and appropriately toughened the U.S. response to Russia’s human rights violations. A statement last weekend condemned the suppression of protests in favor of opposition leader Alexei Navalny, and called on the regime of Vladimir Putin to “fully cooperate with the international community’s investigation” into Mr. Navalny’s poisoning. That, of course, is unlikely to happen. But the new administration may have a readily available means to advance knowledge of Russia’s poisoning operations and hold its regime accountable.

We say “may have,” because, for mysterious reasons, the FBI has refused to reveal what it knows about attacks on another Russian opposition activist, Vladimir Kara-Murza. Mr. Kara-Murza, a permanent U.S resident and Post contributing columnist who divides his time between Moscow and Northern Virginia, has twice suffered apparent poisoning attacks while in Russia, in 2015 and 2017. In both cases, he experienced multiple organ failures and only barely survived. Following the second episode, Mr. Kara-Murza’s wife hand-carried a sample of his blood to Washington. She was met at Dulles International Airport by FBI agents, who took the sample for testing at the agency’s labs.

FBI reports say that the “sum total of the symptoms and health effects Kara-Murza experienced could not have been brought about without a toxin being introduced to his system” and that the bureau is “investigating this matter as a case of intentional poisoning.” But it has refused to release the results of its laboratory tests, which might show whether Mr. Kara-Murza, like Mr. Navalny and other Kremlin targets, was attacked with a banned chemical weapon. Requests from several members of Congress for the results — including Sen. Marco Rubio (R-Fla.) a senior member of the Intelligence Committee — received no response.

We know about the FBI’s conclusions only because of a Freedom of Information Act request and subsequent lawsuit filed by Mr. Kara-Murza in an attempt to obtain documents about his case. The bureau eventually released 1,200 pages of records, but is still withholding 251 on the grounds that they have been “referred to other agencies” for review. Mr. Kara-Murza and his lawyer believe that the missing documents likely include the tests of his blood samples by the FBI lab, and possibly by other U.S. national labs specializing in chemical weapons.

The refusal to disclose is difficult to fathom, especially given the timely and full reports by British and German authorities of evidence of Russian poisonings in the cases of Mr. Navalny and Sergei Skripal, a former Russian double agent attacked in Britain in 2018. Both were poisoned with Novichok, a chemical weapon developed by the Soviet Union that is banned by a treaty to which Russia is a signatory. U.S. legislation mandates sanctions for violations of the pact.

It’s possible that the FBI was unable to determine the poison used against Mr. Kara-Murza; a 2018 report says its lab had been “unable to definitively link a toxin to the victim’s illness.” But the agency has a record of reluctance to acknowledge what it knows about Russia’s possession of banned chemical weapons and their use. In light of the string of attacks on Kremlin opponents and the imperative of holding the Putin regime accountable, that’s not acceptable. Incoming Attorney General Merrick Garland should order the FBI to disclose what it knows about Mr. Kara-Murza.

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