Ryan Smith, a junior varsity defensive lineman at Edmond North High School in Edmond, Okla., died Wednesday night, district spokeswoman Susan Parks told The Oklahoman.
Smith had broken his leg during practice Tuesday and was taken by ambulance to the University of Oklahoma Medical Center in Edmond, where he was treated and released that night.
Parks said Smith was "very lethargic" Wednesday and his parents were unable to wake him.
They called 911 and he was taken to Integris Baptist Medical Center, where he died sometime after 11 p.m., The Oklahoman said.
"We do not know the cause of death," Parks said. "But we do know he was cleared physically to play football (before the season), and there wasn't a concussion or anything when the leg was broken."
The school district has sent counselors to visit with Smith's classmates, teammates and coaches at Edmond North and to Edmond Santa Fe High School, where Smith attended his freshman year.
"Our heart goes out to the family," Parks said. "This is a tragic situation."