MOSCOW (AP) — A private foundation says one of its employees, a prominent Russian opposition activist, has been hospitalized after a sudden illness reminiscent of a poisoning he suffered two years earlier.

Open Russia, run by exiled Kremlin critic Mikhail Khodorkovsky, said in a statement on Thursday that Vladimir Kara-Murza was hospitalized earlier in the day after becoming ill. Kara-Murza, a journalist and a close associate of the murdered opposition leader Boris Nemtsov, nearly died when he suffered sudden kidney failure in May 2015.

No cause for that illness has been determined but Kara-Murza underwent tests showing he had ingested a poisonous substance. In light of the fatal poisoning of defector Alexander Litvinenko and the mysterious deaths of other Russian opposition figures, some worried that Kara-Murza could have been deliberately poisoned.