A UNLV student tragically passed away just days after taking part in a charity boxing event for his fraternity ... and now the incident is under investigation.
20-year-old Nathan Valencia -- a member of the Sigma Alpha Epsilon fraternity -- was part of the main event for the Kappa Sigma-hosted "Fight Night" on November 19 ... when he collapsed shortly after his match with a fellow student.
Valencia -- who had no prior boxing experience -- was taken to a nearby hospital with critical injuries ... where he passed away four days later.