Yehoshua Hefetz, a 19-year-old soldier from Jerusalem, died yesterday morning after collapsing during a training exercise at the Combat Engineering Corps school in the Negev. He will be laid to rest at Jerusalem's Mount Herzl today, in a military ceremony. Hefetz, who was promoted to the rank of corporal after his death, was taking part in an group training exercise for the corps' Sayeret Yahalom special operations unit.

Hefetz collapsed suddenly, apparently due to cardiac arrest, between 7 A.M. and 8 A.M., during a training run rated as intensive but not requiring exceptional physical effort. A knowledgeable medical source who did not want to be identified told Haaretz there was a "brief delay" before the base's medical staff initiated resuscitation efforts.

Military officials said medics were on the scene, as they always are for such training exercises, and tried to revive Hefetz for about an hour. He was then taken by military vehicle, attended by an army physician, to Eilat's Yoseftal Medical Center, a 30-minute drive. Resuscitation efforts continued during the trip, the officials said, in the course of which Hefetz was transfered to an intensive care ambulance.

Doctors at Yoseftal resumed resuscitation efforts but declared Hefetz dead about 20 minutes after his arrival. A postmortem echocardiogram performed at the hospital did not show evidence of a congenital heart defect, but the army's initial investigation points in that direction.

"The big unknown variable is the length of the delay between the collapse and the start of CPR," the source said. Army officers said the weather yesterday was taken into consideration when the training exercise was approved.

The Israel Defense Forces launched two separate investigations into the incident: Maj. Gen. Sami Turgeman, head of the Ground Forces Command, will appoint a panel headed by an officer of the rank of colonel. IDF Chief Medical Officer Brig. Gen. Dr. Yitshak Kreiss has begun an investigation to determine the cause of death.

Dr. Jacob Arad, director of Yoseftal's emergency department, said the description of the incident was not congruent with heatstroke. "The hypothesis is that it was a cardiac arrhythmia, as in cases when basketball players collapse on the court," Arad said.

Hefetz is survived by his mother and two sisters.