The Boulder County Coroner's Office today ruled that the woman who died after collapsing at the Boulder Spring Half Marathon at the Boulder Reservoir in May had an irregular heartbeat and said the runner may have had a condition that caused her heart muscles to thicken.

Boulder County Coroner Emma Hall ruled that Jessica Dillon, 20, died of "cardiac arrhythmia of uncertain etiology with other contributing condition of cardiac hypertrophy." Hypertrophy is a thickening of the heart muscles that can cause a decrease in the size of the heart chamber.

Hall declared the manner of death to be natural, according to a news release sent out today.

Dillon, of Castle Rock, collapsed on May 4 at Boulder Reservoir during the half marathon. She was transported to Boulder Community Health, where she died later that day.

Dillon was listed as a health and exercise science major on Colorado State University's student directory.

Contact Camera Staff Writer Mitchell Byars at 303-473-1329, or