Brian Smart, the Upper Darby High School athletic trainer who died after he collapsed during the Broad Street Run in May, suffered a sudden cardiac death of unknown origin, according to an autopsy completed this month.

Upper Darby school officials said in May that Smart, 25, appeared to have died of a heart attack, meaning a blockage in an artery that cuts off the flow of oxygenated blood to the heart.

But a heart attack is just one of many conditions that can lead to a sudden cardiac death. The actual cause in this case is unclear, said James Garrow, a spokesperson for the Philadelphia Medical Examiner’s Office.

Smart’s death shook his school community, which honored him in a ceremony last Friday before the varsity football game.

When he collapsed after Mile 8 of the 10-mile race in May, it was a reminder that even athletes in good shape can be the victim of an underlying heart condition. An avid runner, Smart ran the 2017 Broad Street Run in 1 hour, 20 minutes, 39 seconds, an average of just over eight minutes per mile.

In young people, sudden cardiac death is rare and can result from various conditions, such as an irregularity in the electrical signals that coordinate the heart’s pumping action or one of several cardiomyopathies — abnormalities in the heart muscle. In certain cases, a genetic mutation is involved.

Determining which is the culprit is challenging, said pathologist Gregory McDonald, dean of the School of Health Sciences at Philadelphia College of Osteopathic Medicine. In a living person, problems with the heart’s electrical function can sometimes be detected with an EKG, but this is not an option after death.

Sudden cardiac death also can be caused by a heart attack, generally in older people, said McDonald, who used to work for the Philadelphia medical examiner. This occurs when the blockage in a coronary artery leads to a fatal irregular heartbeat.

But because the Medical Examiner’s Office did not list myocardial infarction — the technical term for a heart attack — on Smart’s death certificate, that suggests that there was no blockage and that his heart stopped for some other reason, McDonald said.

When Smart collapsed, emergency responders were on the scene immediately and attempted to revive him, Upper Darby athletic director Frank Nunan said at the time. But they were unsuccessful, and Smart was pronounced dead after being transported to the Hospital of the University of Pennsylvania.

Family members did not respond to a request for comment on the autopsy findings. Bill Smart, his father, told CBS in May that preliminary tests indicated the young man had an enlarged heart.

At least two other entrants in the Broad Street Run have died in the race’s 40-year history.

In 1998, Richard Lagocki, 45, of Port Richmond, died after collapsing near the halfway point of the 10-mile race, despite attempts to revive him by a cardiologist and a nurse who also were competing.

In 2007, Robert Massaroni, 29, a champion martial-arts instructor and seventh-grade teacher at Bensalem’s Shafer Middle School, collapsed and died shortly after he finished the 10-mile course.