Northern Virginia medical examiners said yesterday that Fairfax County high school football player Jon Walsh collapsed and died on the training field because of an unidentified abnormality of the heart.

But more than two weeks after the boy's death, examiners said they had closed the case without determining exactly what caused heart failure in the athletic 16-year-old Robinson High School student.

The deputy state medical examiner, Dr. James Beyer, labeled the cause of death "acute cardiac dysfunction." Put simply, Walsh's heart stopped beating.

"The reason for the dysfunction couldn't be identified by an autopsy," Beyer said.

Walsh's coaches and teammates were stunned when he died Aug. 10 during the first day of football practice. Walsh, starting practice after participating in a wrestling clinic in Iowa, had just finished a 40-yard dash and was two-thirds through a 1 1/2-mile run on the track when he collapsed.

Efforts to revive the youth by Coach Ed Henry, his staff and rescue squad medics failed. School officials described the 6-foot, 185-pound Walsh as a "fine physical specimen, one of the best at the school."

Walsh, an offensive lineman, had undergone a thorough physical check-up the day before he died. He was the son of Navy Cmdr. Robert E. and Carol Walsh of 10120 Dundalk St. in the Kings Park West section of Springfield. He would have been a senior this year.

Last year, 13 high school athletes died as a result of participation in football -- nine from injuries that resulted from physical contact and four as a result of heatstroke or heart attack, according to a survey by the American Football Coaches Association.