“His father told me that when he removed his gloves, everything seemed fine,” Quijano said. “And he even told him something like, ‘I think I won, Dad,’ and he was perfectly lucid.” But shortly thereafter, Santillán told an assistant that he was feeling dizzy.

“It was a regular fight,” his father, Hugo Santillán, told cable news channel Todo Noticias on Monday. “We thought this had something to do with his pressure or something to do with the way his nose had bled, but we did not think it was that critical.”

The medical team that treated Santillán saw no reason to believe Santillán had brain injuries before the fight.

“There was no blood in the brain,” Quijano said. “Unless this kid had some form of brain malformation, the main presumption, the most likely and most logical one, is to think that this had to do with the punches he received during the fight.”

The two deaths in close proximity prompted fans and followers to consider the implications for the inherently violent sport.

Lou DiBella, a U.S.-based boxing promoter, said on Twitter that Santillán’s collapse was “unacceptable, HAUNTING, and sickening to watch.” He added: “This has been a terrible week. As a sport/industry, we have to look in the mirror....I have to look in the mirror...”