Mitchell Cook, 14, collapsed Wednesday as his freshman team practiced for Canyon High School in Anaheim Hills. Coaches administered CPR, but Cook later died at a hospital.
According to the Orange County Register, Cook had a mitral valve prolapse that prevented one of his heart valves from sealing tightly, but the condition was not considered life-threatening. The Mayo Clinic says the condition usually doesn't require any lifestyle changes.
The boy's father, Jim Cook, says the teen made regular doctor visits, and there had been no signs of trouble.
A vigil is planned Saturday night at Canyon High School from 7 p.m. to 9 p.m.
Canyon High School begins its school year on Aug. 21.
The Associated Press contributed to this report.