Russian opposition leader Alexey Navalny is in a coma in a hospital in Siberia after suspected poisoning, his spokeswoman said on Thursday.

Navalny, 44, a vociferous critic of President Vladimir Putin, fell ill on a flight from the Siberian city of Tomsk to Moscow this morning, Kira Yarmysh wrote on Twitter.

She said the plane made an emergency landing in the city of Omsk and Navalny was rushed to a hospital, where he is now on a ventilator in intensive care.

Navalny’s relatives suspect that something was mixed in the tea he drank this morning, according to Yarmysh.

She added that doctors have said a toxic agent would have been absorbed faster through the hot liquid.

Last year, Navalny, head of Russia’s opposition Progress Party, fell ill while being held at a Moscow detention center.

His team suspected he was poisoned but doctors said it was a severe allergic reaction.

Navalny did not accept the findings and filed a legal complaint over his alleged poisoning.