The Russian Boxing Federation says it has opened an investigation into the death of Maxim Dadashev following injuries he suffered in an IBF light-welterweight fight against Subriel Matias in the United States on Friday.
The bout was stopped by Dadashev's trainer Buddy McGirt at the end of the 11th round and the 28-year-old was taken to hospital with bleeding on the brain but failed to recover following emergency surgery.
The Russian Boxing Federation said Dadashev's condition worsened after the fight and doctors diagnosed a cerebral edema. It said he underwent a "difficult" operation, but on Tuesday "his heart stopped".
Umar Kremlev, the organisation's secretary general, said an investigation had now begun "into the circumstances that led to the death", claiming "there was some kind of violation".
"We lost Maxim Dadashev today," he said on the federation's website. "Our pupil. We will fully support his family, including financially. We will complete the investigation into the circumstances surrounding this fight, we need to know the truth about what happened.
"This happens in any sport. I think some human factor intervened, there was some kind of violation. We will be always near his family, we will not leave them."
Dadashev had won all 13 of his previous fights before the bout with Puerto Rican Matias and, after receiving heavy punishment in the latter rounds, McGirt called a halt to proceedings at the MGM National Harbor in Maryland.
"It just makes you realise what type of sport we're in, man," McGirt told ESPN.
"He did everything right in training, no problems, no nothing. My mind is like really running crazy right now. Like, what could I have done differently? But at the end of the day, everything was fine (in training).
"He seemed OK, he was ready, but it's the sport that we're in. It just takes one punch, man."
Dadashev's promoters Top Rank paid tribute to Dadashev, saying on Twitter: "Maxim was a talented fighter inside the ring and a loving husband and father outside the ropes. Our thoughts and prayers are with his family and friends."
Former featherweight world champion Carl Frampton tweeted: "Saddened to hear about the passing of Maxim Dadashev. Deepest condolences to his friends and family. RIP."
Promoter Eddie Hearn added: "So terribly sad to hear the news of the passing of Maxim Dadashev. Rest in peace."