CNN  — 

A Russian politician died of – as of yet – unknown causes after falling ill on a plane on Saturday, the latest in a string of mysterious deaths among Russian elites.

Russia’s Deputy Minister of Science and Higher Education, Pyotr Kucherenko, 46, died while returning from a trip to Cuba on Saturday, according to the ministry.

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

Kucherenko’s family said his death may have been from a heart condition but a forensic examination will be held on Wednesday, according to the state-run broadcaster Zvezda.

Speaking to reporters on Tuesday, Kremlin spokesperson Dmitry Peskov said he was not aware of Kucherenko’s cause of death.

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

“Save yourself and your family. Leave as soon as possible. You cannot imagine the degree of brutalization of our state. In a year you won’t recognize Russia at all. By leaving you are doing the right thing,” Super quoted Kucherenko as saying.

Russia's Deputy Minister of Science and Higher Education Pyotr Kucherenko died while returning from Cuba.

Super said he asked Kucherenko if he too wanted to leave Russia, to which, he said, the minister replied: “It is no longer possible to do so. They take away our passports. And there is no such world where they will now be happy with the deputy Russian minister after this fascist invasion.”

The journalist added that Kucherenko told him he’d been taking antidepressants and tranquilizers, quoting him as saying: “I drink them in handfuls. And it doesn’t help much. I hardly sleep. I feel terrible. We are all taken hostage. Nobody can say anything.