An investigation is underway after a lifelong New England Patriots fan died after "an incident" at the Patriots vs. Miami Dolphins game at Massachusetts' Gillette Stadium, officials said.

Dale Mooney, a 53-year old New Hampshire man, died "following an incident" during Sunday night's game, according to the Norfolk District Attorney’s Office. Responders found Mooney "in apparent need of medical attention" and he was taken to a hospital, prosecutors said.

Witness Joseph Kilmartin told Boston ABC affiliate WCVB that Mooney "basically grabbed another fan. ... They started tussling around for a few minutes. At one point, another fan walked over. He punched him ... and [Mooney] just went out."

"It was pretty hard to watch," Kilmartin said.

An autopsy has not been released.

The district attorney's office said Monday that Mooney's death "remains under active investigation" and no charges have been filed.

Mooney's wife, Lisa Mooney, told WCVB, "I want to know what happened. ... What caused this?"

Gillette Stadium officials said in a statement, "We are heartbroken to learn of the tragic passing of Dale Mooney, a lifelong Patriots fans and 30-year season ticket member."

"We continue to work with local authorities to assist them with their ongoing investigation," stadium officials said. "We extend our sincerest sympathies and heartfelt condolences to Dale’s family and to all those who are mourning his loss.”