El Dorado County authorities say an 11-year-old boy who collapsed and died during a youth football practice had an undiagnosed heart defect.

An autopsy conducted Tuesday on Andrew Jacob Lamphere determined the cause of death was an uncommon heart condition known as idiopathic hypertrophic subaortic stenosis or hypertrophic cardiomyopathy (HCM).

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HCM, which is not routinely checked for by physicians, causes the heart muscle to thicken. Over time, the thickening makes it harder for blood to leave the heart, forcing the heart to work harder to pump blood, the National Institutes of Health said on its website.

Authorities say Andrew was participating in the first practice of the season at Golden Sierra High School in Garden Valley when he collapsed just before 8 p.m. Monday. Paramedics found him in full cardiac arrest and took him to a Placerville hospital, where he was pronounced dead.

The team had just started doing non-contract drills when the boy collapsed. Andrew's father is the coach and was on the field at the time.

The Associated Press contributed to this report.