NEW YORK — Elite runner Ryan Shay died of natural causes after collapsing during the U.S. men's marathon Olympic trials, the New York City medical examiner said today.

Shay died of an irregular heartbeat due to an enlarged heart. His heart also had old scarring, but its cause could not be determined, according to Ellen Borakove, a spokeswoman for the medical examiner.

The 28-year-old collapsed in Central Park on Nov. 3, about 5 1/2 miles into the race, and was pronounced dead at a hospital.

His father, Joe Shay, said previously that Ryan was diagnosed with an enlarged heart at age 14. But doctors had repeatedly cleared him for competition, because having a larger than normal heart is not unusual among elite athletes. Training hard in aerobic sports, such as cycling, running or swimming, tends to result in a bigger heart that pumps more blood throughout the body.

Dr. Douglas Zipes, a spokesman for the American College of Cardiology who studies sudden deaths in athletes, said previously it can be difficult to differentiate a normal athlete's heart from potentially deadly hypertrophic cardiomyopathy.

Cardiac echo tests and electrocardiograms can help evaluate whether the heart is healthy or not, said Zipes, a distinguished professor at the Indiana University School of Medicine. Genetic testing can also determine whether a person is at risk for certain problems.

Still, those precautions may not catch everything.

After his son's death, Joe Shay said doctors could not adequately test Ryan using a treadmill when he was a teenager because his heart rate was so low. Zipes said that's not uncommon among elite athletes.