The Atlantic Council has confirmed that John Herbst, one of its senior staffers and a former U.S. ambassador to Ukraine, experienced symptoms consistent with poisoning two years ago.
The statement by the Washington, D.C.-based think tank followed a report that Herbst was among at least four people who experienced alleged poisonings or break-ins by unknown individuals suspected of being linked to the Russian intelligence services over the past two years.
The Atlantic Council said in a statement late on May 16 that Herbst, who has served as U.S. ambassador to Ukraine and Uzbekistan, fell ill in April 2021 and experienced symptoms "that could have been consistent with poisoning, including elevated levels of toxins in his blood."
The statement confirmed a report earlier in the day by the news outlet Agentstvo that cited sources as saying Herbst; Natalia Arno, the U.S.-based chief of the Free Russia Foundation; and a self-exiled Russian journalist, whose identity was not disclosed, suffered similar episodes.
Agentstvo also reported that unknown intruders unlocked a hotel room of Christo Grozev, a Bulgarian investigative journalist with the Bellingcat group, in Montenegro in 2022 and stole his phone.
All of those mentioned in the Agentstvo report have been strong voices in opposition to Russian President Vladimir Putin, and later of Russia's full-scale invasion of Ukraine in February 2022
The Atlantic Council said medical professionals treated Herbst "effectively at the time but could not definitively conclude there was poisoning involved."
Federal law enforcement was also brought in on the matter, and a blood sample from Herbst was taken, though lab results failed to detect toxic compounds.
"We were in touch with authorities immediately at the time of Ambassador Herbst's illness, but due to the results of the test we decided not to make the incident public," said Frederick Kempe, president and CEO of the Atlantic Council.
Herbst has since returned to full health, the council added.
Arno confirmed on May 16 that there were suspicions she may have been poisoned, "possibly by some nerve agent," after falling ill during a recent trip to Europe.
Arno, who previously kept silent about what she experienced during a trip to Prague in early May, wrote about the ordeal on Facebook after the Agentstvo story on the spate of critics of Putin who have fallen ill after leaving Russia.
"There is a suspicion that during my recent trip to Europe I was poisoned, possibly by some nerve agent, investigated by one (or maybe more) Western intelligence agency, I still have neuropathy symptoms but overall I feel much better," Arno wrote on Facebook.
"Russians who had to leave Putin's Russia, but who abroad continue to fight firmly and decisively against the war, against Putin's regime and for a free and democratic Russia, need to understand that they enemy has long tentacles, that there is the possibility of being exposed to danger outside of Russia, so we must always remain vigilant," Arno wrote.
Agentstvo also reported from sources that a self-exiled Russian journalist experienced possible poisoning symptoms while attending a conference of exiled Russian opposition politicians and activists in Berlin in April. The journalist confirmed experiencing the illness, but did not elaborate further, the investigative media outlet said.
Over the past two decades, a series of poisonings, inside and outside of Russia, have targeted dissidents, former intelligence agents, and opposition activists.
The Kremlin has steadfastly denied any links to the suspicious poisonings involving Putin's critics, such as opposition politicians Aleksei Navalny and Vladimir Kara-Murza -- both of whom are currently in prison.
Navalny fell violently ill on an airplane in Siberia before eventually being transported to Germany to be treated for what European labs defined as a poisoning using a Novichok nerve agent in 2020.
Kara-Murza fell deathly ill on two separate occasions in Moscow -- once in 2015 and again in 2017 -- with symptoms consistent with poisoning. Tissue samples smuggled out of Russia by his relatives were turned over to the FBI, which investigated his case as one of "intentional poisoning."
U.S. government laboratories also conducted extensive tests on the samples, but documents released by the Justice Department suggest they were unable to reach a conclusive finding.
In 2018, former Russian spy Sergei Skripal and his daughter were poisoned with a Novichok nerve agent in Salisbury, England.
British authorities say Russia's GRU military intelligence was responsible for the chemical attack and have charged three of its agents with committing the poisoning.