Police and safety personnel responded to the upper deck at Gillette Stadium shortly before 11 p.m. Sunday and found Dale Mooney, 53, of Newmarket, New Hampshire, “in apparent need of medical attention,” the Norfolk County district attorney’s office said in a statement Monday.

Mooney was taken to a hospital, where he was pronounced dead.

“Our investigation has included numerous law enforcement interviews and the examination of multiple angles of video capturing the scuffle prior to Mr. Dale Mooney’s collapse during the Sunday night game at Gillette Stadium,” the district attorney’s office said in a statement.

“Preliminary autopsy results did not suggest traumatic injury, but did identify a medical issue,” according to the statement. “Cause and manner of death remain undetermined pending further testing.”

The district attorney’s office did not provide any details on Mooney’s medical issue.

Joey Kilmartin, who was at the game, said he saw Mooney, a Patriots fan, jawing with a Dolphins fan. The Dolphins won the game.

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“As soon as he walked over, they started tussling,” said Kilmartin, adding that he shared video that he shot of the fight with police.

“There were some people trying to pull them apart and, as they tried to pull them apart, a Dolphins fan punched Mr. Mooney,” he said. “That is when he fell back in the seat and was clearly unconscious. You could definitely tell there was more than a knock out. There was definitely a medical issue happening with his body.”

As the man lay motionless, Kilmartin said, stadium staff and police rushed over and emergency medical technicians came over a few minutes later. The Dolphins fan, who threw the punch, was escorted away by police, he said.

Mooney’s wife, Lisa Mooney, said she feels numb and wants answers about how her husband died.

“I just can’t even believe this is for real,” she told WCVB-TV. “I want to know what happened. What caused this?”

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.

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

Fan violence at sporting events, especially among rival supporters, is a long-running problem. In 2011, paramedic and San Francisco Giants fan Bryan Stow was beaten in the Dodger Stadium parking lot.

Two Los Angeles Dodgers fans punched Stow, who fell to the pavement and suffered severe head injuries. He was awarded about $14 million when a jury found the team was negligent. The attackers received prison sentences.

“People fighting in the stands has become too common nowadays,” Kilmartin said. “People think it’s almost acceptable.”

It didn’t shock Kimartin to see the fight break out at the game, but it was disappointing, he said.