MOSCOW — Before his flight to Moscow departed, Alexei Navalny sipped a cup of tea at a cafe in Russia's Tomsk airport. Later, the prominent Kremlin critic was moaning in pain, prompting an emergency landing with an ambulance waiting to race him to intensive care.

The drama Thursday in Siberia left Navalny in a coma and doctors offering scant public information about the cause. But his spokeswoman and others had no hesitation, speculating that the 44-year-old activist was the latest foe of President Vladimir Putin to be poisoned — perhaps in the tea he drank at the airport cafe.

Even with many questions still unanswered, the stricken Navalny quickly became a rallying point for Russian opposition groups and others who accuse Putin’s government of plots to silence its foes, including similar poisonings in the past.

Navalny’s possible poisoning also could open new rifts with the West, where Navalny is among the best-known opposition figures in Russia. A German medical team was ready to transport Navalny from Russia for treatment.

Doctors in the Siberian city of Omsk had Navalny on a respirator.

In Washington, President Trump said his administration was looking into the possible poisoning, and national security adviser Robert O’Brien called the incident “extraordinarily concerning.”

“If the Russians were behind this . . . it’s something that we’re going to factor into how we deal with the Russians going forward,” O’Brien told Fox News in an interview.

Navalny once ran for mayor of Moscow and faced arrest multiple times for leading street protests. He also has angered Putin and his oligarch allies with a popular YouTube channel that discloses alleged corruption and extravagant excesses.

Kira Yarmysh, Navalny’s spokeswoman, asserted on Twitter: “This is Putin.”

“Whether he personally gave the order or not, the blame is entirely with him,” she added.

Kremlin spokesman Dmitry Peskov said that an investigation will be opened “if [Navalny] was actually poisoned,” adding that he wished Navalny a “speedy recovery, just like any other citizen of our country.”

For Navalny’s backers, parallels were immediately drawn with other cases of apparent retaliation against Kremlin opponents.

In 2006, Alexander Litvinenko, a former spy and prominent dissident, died of polonium-210 poisoning in London, identifying Putin as responsible while in his hospital bed. The polonium was in his tea.

More recently, Sergei Skripal, a former double agent, and his adult daughter, Yulia, were poisoned in 2018, after they came into contact with a deadly Soviet-era nerve agent known as Novichok.

Pyotr Verzilov, a member of the Pussy Riot protest group, accused Russian military intelligence for his suspected poisoning in 2018. He wrote on Twitter Thursday that both his and Navalny’s alleged poisoning were “certainly sanctioned personally by Putin.”

“In Russia, operations of this level are not carried out without the consent of the first person,” he said.

The chief doctor for the Omsk Emergency Hospital No. 1, where Navalny is in the intensive care unit for toxicology patients, told the state-run Tass news agency that Navalny was in serious condition. Another physician at the hospital, Anatoli Kalinichenko, told local reporters that “there is no certainty that the cause of Navalny’s condition is poisoning.”

“Doctors are working to save his life,” Kalinichenko said, adding that Navalny is “stable.”

Yaroslav Ashikhmin, Navalny’s longtime physician, told the Russian news outlet Meduza that the opposition leader “needs to be taken to Europe” for treatment because a Western clinic had a higher chance of identifying what might have poisoned him. Yarmysh said the Omsk hospital had yet to offer a diagnosis.

An ambulance aircraft with a team specialized in treating coma patients is due to leave Germany to pick up the stricken Navalny, Berlin-based human rights activist Jaka Bizilj told the German Bild newspaper.

But it’s unclear whether Navalny will be released because of possible complications in paperwork to transfer out of the country. When Navalny’s wife arrived to visit him, she was initially denied because she didn’t have a marriage license with her to prove their relationship, Yarmysh said. She was later permitted to see him.

Yarmysh said she suspects a poisonous substance was slipped into Navalny’s tea because it “was the only thing Alexei drank this morning,” she wrote on Twitter.

The manager of the Tomsk airport coffee shop where Navalny drank the tea told the Interfax news agency that he is “looking into all circumstances and studying [security camera] footage.”

In one video, shot during the flight and later posted to social media, Navalny can be heard moaning in pain.

Navalny traveled to Tomsk to meet with activists and opposition candidates for regional elections next month. Navalny and his allies have encouraged voters to back anti-Kremlin candidates as a message of discontent over Russia’s sagging economy and the unchecked power of Putin, who has the potential to stay in office until 2036 under constitutional changes approved this year.

A year ago, Navalny was hospitalized with an “acute allergic reaction” a week after being detained, and at the time, Yarmysh said he may have been affected by an unknown chemical substance, citing a physician who observed Navalny at the hospital. In 2017, Navalny was attacked with an antiseptic green dye that damaged vision in one of his eyes.

Navalny, who was barred from running for president in 2018, has frequently been jailed and harassed. In March, authorities seized the contents of his bank account as well as the accounts of his wife, son and daughter. Last month, Navalny was forced to close his Anti-Corruption Foundation because of a lawsuit filed by oligarch Yevgeniy Prigozhin, a close associate of Putin. The foundation exposed graft and other wrongdoing by Russia’s elite for more than a decade, making Navalny an even bigger target for attacks.

In 2018, the European Court of Human Rights ruled that Russia’s arrests and detention of Navalny from 2012 to 2014 violated his rights and appeared to be part of a broader effort “to bring the opposition under control.” Moscow disagreed with the decision.