The death of a Mississippi high school football player three days after he fell ill during a game was due to swelling in his brain possibly triggered by overhydration, a doctor who treated him said on Tuesday.

“This seems to be a fluke, freak tragedy,” said Dr. Joe Pressler, who treated Walker Wilbanks, 17, at the University of Mississippi Medical Center.

Wilbanks was playing in a Friday night game for Jackson Preparatory School, the state’s top private high school football team, when he came off the field saying he did not feel well, a school spokeswoman said.

He died on Monday from cerebral edema, or an excess of fluid in the brain that caused it to swell, Pressler said in a phone interview.

That condition was triggered by low sodium concentrations in his blood, a result consistent with overhydrating after heavy perspiration, he said.

Both before and during the game, Wilbanks drank Gatorade and Pedialyte, beverages with sodium concentrations that are higher than in water but lower than what is naturally found in the body, Pressler said.

A 2013 study published in the American Journal of Sports Medicine found that an average of 12 high school and college football players died annually during practices and games, with heart conditions, heat and other non-traumatic causes of death twice as common as injury-related ones.