Russian boxer Maxim Dadashev has died at the age of 28 following injuries suffered in his IBF light-welterweight fight against Subriel Matias.
Dadashev was reportedly unable to walk to the dressing room after his bout was stopped by trainer Buddy McGirt at the end of the 11th round on Friday.
He was hospitalised with bleeding on the brain and underwent emergency surgery but failed to recover.
The Russian Boxing Federation says it has opened an investigation.
Secretary general Umar Kremlev suggested there was "some kind of violation", adding in a statement: "We lost Maxim Dadashev. He was our young prospect.
"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 factors intervened, there was some kind of violation."
USA-based Dadashev had won all of his previous 13 fights but had to absorb a barrage of punches from Puerto Rican Matias during the course of the fight in Maryland.
McGirt had said afterwards he "could not convince" his fighter to stop, but opted to throw in the towel when he saw him "getting hit with more and more clean shots as the fight went on".
The Russian Boxing Federation said that after the fight, Dadashev's condition worsened and doctors diagnosed a cerebral edema and a "difficult" surgery took place, but his heart stopped on Tuesday.
Northern Ireland's former world champion Carl Frampton was among those to pay tribute, saying on Twitter: "Saddened to hear about the passing of Maxim Dadashev. Deepest condolences to his friends and family. RIP."
British boxing promoter Eddie Hearn added: "So terribly sad to hear the news of the passing of Maxim Dadashev. Rest in peace."
'I don't think there was a dereliction of duty'
ESPN.com writer Steve Kim, who was ringside at the bout, spoke to BBC World News and said: "Nobody thought that fight should've been stopped really any earlier than it was.
"McGirt was very much lauded for stopping the fight against the wishes of Dadashev after the 11th round. Unfortunately that was a very physical fight for him against Matias, who was strong and was consistently hitting him with hard shots but never really putting him away and it wasn't until the 11th that he started to bend physically.
"I've written extensively in the past, excoriated trainers for being braver than their fighters. I'll be honest I don't think you can second guess or even question the decision made from that corner on the night.
"The referee was Kenny Chevalier, a veteran referee, he's done many championship fights and he's quite prominent in that part of our country.
"Like with McGirt I don't think there was a dereliction of duty."