The man was identified as Kyle A. Vanwinkle, 30, of Smithville. An examination of Vanwinkle's body did not show obvious signs of trauma, Kansas City police spokesman Darin Snapp said in an emailed statement.
Snapp said a man who owned a Jeep and his son returned to the vehicle during Sunday's game and found a man "who did not belong" inside, which led to a fight between the owner and Vanwinkle.
Police responded to a disturbance call in Lot A at Truman Sports Complex, where Arrowhead Stadium is located, and found Vanwinkle on the ground unconscious. Three people were taken into custody Sunday evening, but all were released Monday afternoon with no charges filed.
Snapp said investigators are awaiting autopsy and toxicology reports before deciding whether Vanwinkle's death was the result of a crime.
"We don't know if the person had a health condition, had a heart attack. We don't know," Snapp said Sunday. "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."
It wasn't clear whether the men knew each other, or whether the Jeep owner had been in the stadium or was outside tailgating with other fans. Arrowhead has a no re-entry policy, meaning anyone who left the closely contested game would not have been allowed to come back inside.
Officers searched a suburban Kansas City home Monday morning after investigators learned evidence in the man's death might be found there, Snapp said. He declined to provide further details.
Ted Crews, a spokesman for the Chiefs, said the team was aware of the incident but could not comment further because of the police investigation.
The death came one year to the day after Chiefs linebacker Jovan Belcher shot and 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 fatally shot himself 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 also services Kauffman Stadium. The shooting took place several hours before the scheduled start of the Royals' game, which went on as scheduled.
AP Sports Writer Dave Skretta and freelance writer Geoffrey Calvert contributed to this report.