A man who attended Sunday night's New England Patriots game at Gillette Stadium in Foxborough, Massachusetts, said he witnessed a "violent confrontation" leading up to the death of a fan, according to an interview with CNN affiliate WCVB.

Dale Mooney, 53, from Newmarket, New Hampshire, died after "an incident" during the New England Patriots versus Miami Dolphins game on Sunday night, the Norfolk County district attorney's office said.

Joseph Kilmartin, a Patriots fan who was at the game, told WCVB that he witnessed Mooney approach a group of fans on the 300-level of the stadium toward the end of the fourth quarter.

"He basically grabbed another fan," Kilmartin said. "They started tussling around for a few minutes. At one point, another fan walked over. He punched him ... and the man just went out. It was pretty hard to watch," Kilmartin said.

Kilmartin declined CNN's request for an interview.

Foxborough police and fire initially responded to the incident around 10:57 p.m., when Mooney needed medical attention, the district attorney's office said in a statement to CNN Monday.

Mooney was transported to Sturdy Memorial Hospital where he was pronounced dead just before midnight, the Massachusetts State Police said in a statement to CNN Monday.

Police and the Norfolk County district attorney have opened an investigation. CNN has reached out to the state police and the district attorney's office for comment on the alleged confrontation but has not heard back.

Mooney's wife, Lisa, told WCVB that her husband, a season ticket holder for 30 years and father of two, went to Gillette Stadium with three friends who later told her they were "verbally taunted" by other fans during the game.

"It takes a lot to get him mad," Lisa Mooney said. "He has the patience of a saint. I don't know what happened."

According to Lisa Mooney, her husband had no known medical condition that she was aware of.

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

CNN has been unable to reach Lisa Mooney for comment.

The autopsy results have not been released, and CNN's calls to the medical examiner have not been returned.

CNN has reached out to Gillette Stadium and the New England Patriots for comment but has not heard back.