Kansas City police spokesman Darin Snapp told The Associated Press that the incident was being treated as a homicide but that no arrests had been made. Snapp said the victim had returned to his vehicle to find a stranger inside it and a struggle ensued.
The victim, a man in his 20s, was found lying unconscious on the pavement, Snapp said. Paramedics began administering first aid before the man was transported to the hospital, where he died. His name was not immediately released.
"Talking to several witnesses that were out here in the parking lots, there was an individual who came back to his vehicle, saw someone inside the vehicle that did not belong. A struggle ensued between the two. That's when the individual collapsed," Snapp said.
Snapp said the man had his son with him when he returned to his vehicle and that the boy - whose age was not immediately known - had run for help when the struggle began.
Snapp said there were no obvious indications of how the man died.
"We don't know if the person had a health condition, had a heart attack. We don't know," Snapp said. "We're going to investigate this as a homicide until we hear differently. Right now it's a suspicious death but we are going to investigate it as a homicide."
Chiefs spokesman Ted Crews said the team is aware of the incident but cannot comment further because of the police investigation.
Flashing lights from ambulances and police cars were visible across the Arrowhead Stadium parking lot as cars tried to leave after the game.
The death came one year to the day after Chiefs linebacker Jovan Belcher killed his girlfriend, Kasandra Perkins, on Dec. 1, 2012, at a home not far from the stadium. Belcher then drove to the team's practice facility and committed suicide in front of then-coach Romeo Crennel and general manager Scott Pioli.
In September 2012, a gunman attacked a Kansas City Royals employee in the same parking lot, which services Kauffman Stadium as well. The gunman shot the employee once in the abdomen and then returned to his vehicle and committed suicide.
Snapp said police were speaking to several witnesses after Sunday's incident, and that three people had been taken to the Kansas City Police Department's headquarters for questioning.
"Everyone was very cooperative when we got here," he said. "Everyone that we believed was involved is still here and we're talking to them. Right now they're persons of interest. Like I said, until we find out exactly how this person passed away, we're going to investigate it as a homicide. They're not suspects right now until we find out exactly what happened."
Associated Press writer Geoffrey Calvert contributed to this report.