The Pennington County Coroner's Office has released the autopsy results for 16-year-old Ryan Gramberg, who died in July after collapsing at football practice at Rapid City Stevens High School.

Gramberg died of hypertrophic cardiomyopathy, which is a disease that makes the heart muscle thicken.

The rare condition is associated with people who seem otherwise healthy.

Gramberg passed out on the practice field after a light evening practice July 14 and was taken by ambulance to Rapid City Regional Hospital, where he later died.

An initial coroner's report showed Gramberg had an enlarged heart.

School officials said at the time of the incident that the team had not gone through a particularly strenuous or out-of-the-ordinary practice before Gramberg's collapse.

The team was having its summer team camp.