An investigation has been launched into the death of a mixed martial arts fighter believed to have suffered poisoning from an unidentified substance in Russia.

Aleksandr Pisarev, 33, and his wife were hospitalized after falling ill on October 30, according to various reports. Pisarev died shortly thereafter while his wife remains in hospital.

“Alexander Pisarev died in his sleep and did not have any chronic health problems,” a member of Pisarev’s Tomahawk team told Russian state news agency “According to preliminary data, death was the result of food poisoning.”

The Tomahawk team later confirmed Pisarev’s passing without listing a cause of death.

My brother, my friend, my student! Words cannot express the extent of the loss. Alexander Pisarev is the standard of friendship, decency and courage of a Russian person,” read the statement on VKontakte. “I and we will miss you!!! Rest in peace our brother.”

Local law enforcement in riverside city of Balashikha, which lies approximately 20km east of Moscow, have begun investigating the cause of Pisarev’s death.

According to the Moscow Region Investigative Committee, “investigators, with the participation of a forensic specialist, are conducting an additional inspection of the scene, confiscating items relevant to the investigation, and planning a forensic chemical examination.”

Reports have since claimed that Pisarev suffered food poisoning from a watermelon, while others have suggested the fighter died of opioid poisoning.

Pisarev was a local Moscow champion in hand-to-hand combat, as well as a Russian Central Federal District MMA champion. He competed in various Russian fight leagues, including RCC and Fight Nights.