Fierce Kremlin critic and opposition politician Alexei Navalny was poisoned and hospitalized Thursday morning, his spokesperson said.

The 44-year-old foe of Russia’s President Vladimir Putin felt unwell on a flight back to Moscow from Tomsk, a city in Siberia, Kira Yarmysh said on Twitter.

“The plane made an emergency landing in Omsk. Alexei has a toxic poisoning,” Yarmysh tweeted.

Navalny, 44, is said to be unconscious and was placed on a ventilator in an intensive care unit.

Yarmysh did not say who she believed may have poisoned Navalny, but said police had been called to the hospital.

The politician is in a grave but stable condition, hospital representative Anatoly Kalinichenko, deputy chief physician at the Omsk Emergency Hospital No. 1., said in a video shared by Yarmysh on Twitter. Kalinichenko said all possible reasons for Navalny’s sudden illness were being looked at, including poisoning.

“There could be a number of conditions that started suddenly and could have led to the same clinical symptoms,” he said. “We are working on all of these.”

Yarmysh told Russian radio station Echo of Moscow there are tests being conducted to determine the nature of the toxin used. She said Navalny only had a black tea at an airport coffee shop before getting on the plane in the morning, and they believe that’s how he could have been poisoned.

She said she was sure it was “an intentional poisoning.”

“A year ago, he was poisoned in a prison, and I am sure the same thing happened here,” she told the station. “It’s different symptoms, obviously a different toxin, but obviously this was done to him intentionally.”

Last year, Navalny was rushed to a hospital from prison where he was serving a sentence following an administrative arrest, with what his team said was suspected poisoning.

Doctors then said he had a severe allergic attack and discharged him back to prison the following day.

In 2017, he was attacked by several men who threw antiseptic in his face, damaging one eye.

Navalny rose to prominence in 2009 with investigations into official corruption and became a protest leader when hundreds of thousands took to the streets across Russia in 2011 to protest electoral fraud.

A few years later, and after several short-term spells in jail, Navalny faced two separate sets of fraud charges, which were viewed as political retribution aimed at stopping him from running for office.

In his only official campaign before his first conviction took effect, Navalny garnered 30 percent of the vote in the race for Moscow mayor in 2013.

Navalny also campaigned to challenge Russian President Vladimir Putin in the 2018 presidential election, but was barred from running.

Navalny’s anti-corruption foundation has conducted in-depth investigations into the highest ranks of Russian political elite, including his most famous investigation into former prime minister and president Dmitry Medvedev.

Last month, he had to shut down the foundation after a financially devastating lawsuit from Yevgeny Prigozhin, a businessman with close ties to the Kremlin.

Russia holds regional elections next month and Navalny and his allies have been preparing for them, trying to increase support for candidates which they back.

Reuters and The Associated Press contributed to this report.