Vladimir Kara-Murza, a vocal critic of Vladimir Putin’s Kremlin, collapsed in Moscow on Tuesday. A friend of Boris Nemtsov, the Russian dissident murdered in February, the 33-year-old showed no previous signs of illness.

Andrei Bystrov, a friend of the Kara-Murza family and a fellow dissident, recently announced that doctors had confirmed that Kara-Murza had been poisoned. He went on to say that doctors are still identifying the exact poison, telling the Telegraph, “It could be anything.”