KANSAS CITY, Mo. — Nathan Stiles’ parents had little reason to doubt their 17-year-old son was ready to return to the football field after suffering an apparent concussion at the homecoming game. He spent weeks resting, and a doctor gave him the all clear to play again. Even a CT scan that usually detects more serious head injuries came back negative, his mother said.

The Spring Hill High School senior died in late October, a day after collapsing on the sidelines at a game. A county coroner determined this week that the running back and linebacker died of a rare but severe brain injury that likely occurred during the earlier game but went undetected.

The suburban Kansas City school district says it followed new rules designed to reduce concussion risks. But in an era of heightened awareness about player safety and head injuries, family, school and medical experts are left wondering if anything more could have been done to prevent his death.

“It’s tragic,” said Rick Bowden, assistant executive director of the Kansas State High School Activities Association. “But I don’t know if anything could have been done differently. When a doctor releases a student to return to participate, what more can we ask?”

On Oct. 28, Stiles scored at least one touchdown in Spring Hills’ 99-72 win over Osawatomie. But then he left the field on his own after hitting his head on the ground while being tackled by two opponents late in the second quarter. He collapsed, and a day later he died.

The Johnson County coroner on Thursday ruled that Stiles died of a subdural hematoma, or bleeding between the tissue layers connecting the brain and skull. Medical experts say it’s unclear why the CT scan done after the initial injury didn’t show the bleeding.

Eighty-six percent of the deaths from brain injury in sports over a more than 50-year period through 1999 were due to subdural hematomas, researchers found.