Russian doctors treating opposition politician Alexei Navalny said Friday that they do not believe he was poisoned and refused to let him be transferred aboard a waiting plane from Germany to take him to Berlin for treatment.

The 44-year-old Navalny, one of Russian President Vladimir Putin’s fiercest critics, was admitted to an intensive care unit in a coma at a hospital in the Siberian city of Omsk on Thursday, following what his supporters are calling a suspected poisoning that they believe was engineered by the Kremlin. Russian agents have been implicated in the poisonings, some fatal, of dissidents in the past.

But the Russian doctors treating Navalny insisted Friday that they had not found any indication that the Kremlin critic was poisoned. Omsk hospital deputy chief doctor, Anatoly Kalinichenko, said no traces of poison were found in Navalny’s body.