(NewsNation) — So far, nobody has been charged in the death of 53-year-old Dale Mooney, who died at Sunday night’s New England Patriots game.

A preliminary autopsy report “did not suggest traumatic injury but did identify a medical issue,” according to the Norfolk County District Attorney’s Office. Authorities are still working to determine the exact cause and manner of death.

First responders rushed to the 300 section of Gillette Stadium’s upper deck just after 11 p.m. to help a man needing medical attention. The DA’s office is still investigating what happened, but witnesses say Mooney was punched at least twice in the head during a fight with another fan.

One witness described the ordeal as “horrific” after a “vicious punch” left a man unconscious, according to NewsNation affiliate WPRI.

“It was mostly just tussling then you saw a fan deliver two punches to the victim, a Dolphins fan, he was wearing a Dolphins jersey, and the victim just slumped over at that point in his seat,” Joe Kilmartin, who witnessed the fight, told WPRI.

Emergency responders rushed Mooney, a New Hampshire native, to the hospital, where he was pronounced dead.

Gillette Stadium officials said they were “heartbroken” by the death of Mooney, who they described as a lifelong Patriots fan and 30-year season ticket member.

The fight is the latest example of fan violence at a professional sporting event.

Last year, an all-out brawl broke out among fans at a preseason game between the Jacksonville Jaguars and Pittsburgh Steelers. One man was arrested for that melee.

The DA’s office told NewsNation it has examined multiple video angles and is asking for anyone who witnessed the event to come forward with information.