A 15-year-old Northwood High School student died Wednesday after collapsing during a late-afternoon football practice on the Irvine campus, police said.
Authorities did not immediately release his name.
"We have no indication of what caused this," said Lt. Rick Handfield, a spokesman for the Irvine Police Department.
The athlete collapsed, he said, shortly after 5 p.m. at the school on the 4500 block of Portola Parkway.
A sports trainer tried to revive him by administering cardiopulmonary resuscitation but was unsuccessful, Handfield said.
The student was pronounced dead at a hospital.
"It will take an investigation with the coroner's office to piece this together and find out what happened," the spokesman said.
Outdoor temperatures at the time were moderate, he said.
Ian Hanigan, a spokesman for the Irvine Unified School District, described the incident as "devastating."
"Obviously," he said, "our heart goes out to the family and friends and all those who knew him and are suffering a great loss right now."
He said grief counselors would be on campus beginning today to help students and staff members cope with the loss.
A coroner's spokesman said an autopsy would be performed today.