The 35-year-old "complained that he felt bad and was suffocating".
Ivan Sechin, the 35-year-old son of Igor Sechin, an ally of Russian President Vladimir Putin and the CEO of Russian state-owned oil firm Rosneft, died on February 5, as per a report in Newsweek.
The death of Mr Sechin had not been reported until exiled Russian oligarch and Vladimir Putin critic Leonid Nevzlin drew attention to a new record in the national register of inheritance cases. The entry in the registry is registered to Ivan Igorevich Sechin, who was born in 1989 and resided in a building previously identified as "a house for President Putin's associates," according to independent Russian news outlets.
Mr Nevzlin took to Facebook and said that it was a "strange coincidence". He added, "Today, when we recalled with my friends and partners why started the Kremlin war against Yukos, namely Mikhail Khodorkovsky's report on corruption at the RPP meeting, it was today that I learned that Igor Sechin had just lost his son. The very same man who fabricated every blatant and bloody case against us. And still."
"Probably hard to believe but my heart goes out to a father who lost his child. How I sympathize with Alexei Navalny's mother. How I sympathized in recent years to Bory Nemtsov's mother, who left us this month. My condolences to the Sechiny family, who are forced to hide their grief," he continued.
Further, a local Russian outlet stated that the 35-year-old, who was employed by Rosneft, "complained that he felt bad and was suffocating" around 4:30 am on February 5. "The man fell on the bed and lost consciousness. Those around him carried Ivan to the floor and tried to provide primary resuscitation measures," they wrote. The outlet's Telegram channel stated that an ambulance drove for two hours trying to find him. "When the ambulance arrived at the scene, the doctors were only able to ascertain Ivan's death," they wrote, adding that the cause of death was identified as a "detached blood clot."
The outlet further said that Mr Sechin was buried "on the third day after his death. Rosneft's CEO, "prohibited employees of the Investigative Committee, the FSB, and other security forces from dealing with issues of Ivan's death."
Several prominent Russians have died in unexplained circumstances since Russian Mr Putin invaded Ukraine in February 2022. Marina Yankina, a high-ranking Russian defence official who headed the Russian Ministry of Defence's financial support department, died after falling 16 storeys in St Petersburg in February last year.
Marina Yankina's body was discovered by a bystander on the pavement of a residential complex in St. Petersburg's Kalininsky area before 8 a.m. The 58-year-old was a key figure in boosting funding efforts for Mr Putin's war in Ukraine.
She was the finance director of the Western Military District, one of Russia's five geographical battalions, whose commander Mr Putin has changed several times since invading Ukraine.