A Northern Virginia jogger was found dead yesterday and another was feared to have perished in a 10-mile foot race that some medical experts said should have never been held in Sunday's blistering heat.

The crumpled body of Patrick Reiley, a 31-year-old Arlington school teacher, was discovered in a grove of trees about 300 yards from the course of the race sponsored by the town of Herndon. Police said Reiley, a mathematics and physical education teacher running his first long-distance race, apparently collapsed from the heat and humidity.

Searchers and family members feared that also was the fate of the missing runner, Henry J. Kronlage, 49, an IBM engineer from Fairfax County. Kronlage mysteriously disappeared from the pack of 625 runners on the course that ran along remote stretches of countryside near Dulles International Airport.

Sports medicine experts and some jogging association officials yesterday questioned the wisdom of staging a long-distance run in Sunday's searing, humid weather. "It seems that if people are wandering into the woods and dying," said a trustee of the American College of Sports Medicine, "something is amiss."

Late yesterday more than 100 volunteers -- including the missing man's son -- searched the woods near the airport until after darkness fell for Kronlage, who was running a second time in the annual race. The search is to resume this morning.

"I'm upset my Dad is gone, but it's up to God to determine what will happen now," said 18-year-old Jim Kronlage during a break in the search.

Officials in the Herndon Recreation Department, which sponsored the race, disputed statements that they should have delayed the event because of the weather. The temperature at 9:15 a.m. when the race started was 82 degrees in the shade with a humidity reading of 59 percent, said National Weather Service officials at Dulles.

The temperature rose steadily throughout the race, reaching a high of 88 degrees at 11 a.m. when most participants were crossing the finish line.

According to guidelines of the American College of Sports Mediciine, races of 10 miles or more should be run before 9 a.m. or after 4 p.m. whenever temperatures exceed 80 degrees.

"Ten to 2 o'clock are the most dangerous hours," said Gabe Mirkin, a sports medicine specialist in Silver Spring who is a committee member of the college. "That has a great deal to do with the problem. I ran 10-miles Sunday and I was exhausted. But I was going slow. I would never have run in a race."

Reiley, who lived on Freetown Court in Reston, and Kronlage, who lived on Braeburn Drive in Fairfax County, were not missed until several hours after the other runners had completed the winding, rural course.

There were few trees along the course to protect runners from the sun. And those who ran the Herndon race last year said they noticed something else about the heat -- no lawn sprinklers to dash through.

Last year the course went through neighborhoods where spectators generously doused runners with water from hoses and lawn sprinklers. The only refreshment this year came at watering stations, spaced about 1 1/2 miles apart.

Medical experts said that too could have played a role in Sunday's apparent dual tragedy.

"It might make a very significant difference," said Samuel M. Fox, director of the cardiology exercise program at the Georgetown University Medical Center. "The cooling effect of a nice water spray would be very consequential as compared to just the evaporative cooling of your own sweat."

Pat Reiley "was a victim of the elements, the high temperatures and high humidity during the day, plus the fact that this was his first complete race," said his brother, Mike.

Another brother, Barney Reiley, said Pat Reiley "had never run 10 miles before, although he had recently completed eight miles. He ran now and then."

"They probably got very excited about being in competition," said Christine Wells, a heat stress authority and trustee of the sports medicine group. "What happens is they don't want to stop. They want to keep up with somebody or make a certain time and they don't notice they're becoming dehydrated."

Race officials said they did not screen applicants for ability, "but leave that to the judgment of the individual."

"Only several days ago he told me this was the happiest summer of his life," recalled Agnes Johnson, 84, who rented a room from Reiley at his three-bedroom condominium in Reston. "He was supposed to leave this week for a vaction in Alaska."

Kronlage had been jogging regularly for about four years and had encouraged his sons and daughter to take up the sport as well. They said he had been running five miles "almost every day" in preparation for Sunday's event and was determined to run it in 80 minutes, more then 10 minutes faster than his time last year.

"He had run when it was hot," said his wife, Nancy Kronlage. "He loves hot weather. He's a real New Orleans boy."

Kronlage was last seen by runner Brad Rigby, 19, at the midway point on a Dulles Airport service road.

"He was in good shape," said Rigby. "He passed me by. I was starting to feel the heat, and had to walk some myself" he said yesterday while combing the area along with about two dozen other people in search of Kronlage.

During the past four years the Herndon race was held at night.

Kronlage's daughter, 16-year-old Lynne, who also ran in the race and runs cross-country for W. T. Woodson High School, said she nearly fainted when she arrived home Sunday. "I've never felt so sick after a race," she said.

Herndon recreation director Arthur A. Anselene said, however, that "if anything changing the time and course of the race should have helped. We had better traffic control, and just as many people sprinkling water on the runners.

"It's just a shame they failed to make contact with anyone," Anselene said."A rescue unit could have been there in minutes."

Race participants and police were mystified as to how two people surrounded by hundreds of others could leave the road and not be seen by anyone. Herndon Police Chief Walter R. Bishop said, "It's hard to know what occurred. We assume both men were looking for shade. But in the case of Mr. Kronlage, because he hasn't been found, we wonder if he might have gotten delirious and wandered off. We just don't know."

Sports medicine authorities speculate that that is precisely what happened.

"Your judgment goes off," said Christine Wells, a heat stress expert at Arizona State University. "You become quite delirious."

The search for Kronlage at various times involved a helicopter from the U.S. Park Service, tracker dogs, on-duty and volunteer police officers from several Northern Virginia jurisdictions and dozens of civilian volunteers.

Several small streams run near the airport, and officials yesterday were concerned that Kronlage's body might be covered up by water. The area received three-tenths of an inch of rain Sunday night, a Herndon Public Works official said.

Though hundreds of persons die jogging each year, Sunday's incident was one of the worst cases involving a sponsored race in the Washington area. Some running enthusiasts feared yesterday that the deaths could affect future races.

"It's a frightening situation" said Mirkin, an avid runner and a member of the sports medicine committee for the Roadrunners Club of America. "Something like this could ruin distance running in the area forever."