Russia’s deputy science minister, 48-year-old Pyotr Kucherenko, died mysteriously when he suddenly fell ill during a flight from Cuba to Russia, the ministry said.

The flight made an emergency landing in an area in the south of the country, but the doctors who were called were unable to save his life.

“Kucherenko felt ill while on the plane with a Russian delegation returning from a business trip to Cuba. The plane landed in the town of Mineralnye Vody, where doctors tried to help,” the ministry said in a statement posted on its website, adding that the minister could not be saved.

The mysterious death of the Russian politician who died months after he allegedly made comments critical of Russia’s invasion of Ukraine adds to the long list of mysterious deaths of Russian oligarchs over the past year.

Kucherenko’s family said his death may have been due to a heart condition, but a post-mortem examination to be held on Wednesday, according to state broadcaster Zvezda, is expected to shed light on the cause of his death.

Journalist Roman Super, who fled Russia shortly after it invaded Ukraine last February, said on his Telegram channel that he had spoken to Kucherenko “a few days” before he died. He said Kucherenko feared for his safety because of this and encouraged him to leave Russia.

At the same time, he revealed that when Super asked Kucherenko if he also wanted to leave Russia, the deputy minister replied that it was no longer possible to do so. “They are taking away our passports.” The reporter added that Kucherenko told him he was taking anti-depressants and tranquilizers, quoting him as saying: “I drink by the handful. And they don’t help much. I hardly sleep. I feel awful. We are all hostages. Nobody can say anything. Otherwise, we are immediately crushed like bugs.”