The deaths of three runners who collapsed during a 13.1-mile half marathon appear to have been an aberration, but at least six runners have died while competing in such events in the last two months.

Autopsies were inconclusive Monday on the bodies of Rick Brown, 65, of Marietta, Ohio; Daniel Langdon, 36, of Laingsburg in central Michigan; and Jonathan Fenlon, 26, of Waterford, northwest of Detroit. The Wayne County medical examiner has requested toxicology tests.

The three died Sunday during or after running a half marathon at the Detroit Free Press/Flagstar Bank Marathon. Brown and Langdon collapsed near the end of the race, and Fenlon died after crossing the finish line. Friends and family of the men said they had trained for the 13.1-mile race and were in great shape.

Their deaths follow those of two runners in their mid-thirties who died earlier this month while running a half marathon in San Jose, Calif., and a 23-year-old man who died in September after collapsing during a half marathon in Virginia Beach, Va.

"This is definitely atypical. In a given year, you're looking at four to six deaths," said Ryan Lamppa, a researcher for Colorado Springs-based nonprofit Running USA. "Some years are higher. Some years are lower."

Fewer than one in 10,000 runners who finish a marathon dies, said Lamppa, who did not immediately know the rate of death for overall participants in full or half marathons.

All of the runners in Sunday's races, which included full and half marathons, had to sign a medical release form and were encouraged to talk to their doctors before participating, said Rich Harshbarger, vice president of consumer marketing for the Detroit Media Partnership, which organized the race.

Making sure runners follow those precautions is something that can't be "policed," Lamppa said.

"Almost all of the runner deaths we hear about, it's always described as he or she is in great health," said Jean Knaack, executive director of Arlington, Va.-based Road Runners Club of America, a nonprofit promoting long distance running.

"Even if they are runners and are doing all the right things, they might have a genetic heart condition," Knaack said. "No matter what, whenever you start an exercise program or whether you are a beginning runner, you should consult with your doctor. Regular runners need annual physicals."

Participation in full and half marathons has been growing over the past 10 to 15 years, Lamppa said.

In 2008, an estimated 425,000 runners completed full marathons in the United States, and 715,000 finished half marathons, he said.

The half marathon in Detroit drew about 8,500 runners last year. On Sunday, about 9,000 signed up. About 4,500 ran in the full marathon this year compared to 4,259 in 2008.

The last death at the Detroit event was in 1994, when a 42-year-old man died of a heart attack after running more than 20 miles.

Sunday's three deaths came as a shock to friends and family of the men, who described them as in good shape.

Brown, the oldest of those who died, usually ran the full Detroit marathons, but decided to join his wife in the shorter event, his friend Chip Allman said.

"He'd had some health problems which weren't related to running. He wasn't in the best of shape," said Allman, president of the River City Runners Club in Parkersburg, W. Va. "He'd run some half marathons already this year and did well in them."

Langdon had run half marathons before, but hadn't trained for a full marathon, said his mother-in-law, Deborah Windish.

Fenlon jogged and weightlifted, was healthy and had no history of heart-related ailments, said his mother, Laura Fenlon.

Fenlon had run shorter races than a half marathon, including about four miles in last year's relay portion of the Detroit marathon and had been training with his girlfriend since June for Sunday's race, said his mother, Laura Fenlon.

"They had been running like six miles," Laura Fenlon said.

Lamppa said a runner should average between 50 and 70 miles per week when training for a full marathon, and 25 to 50 miles each week for the shorter version.

"I wouldn't take running the half marathon lightly," said Dr. Stephen Pribut, who has run two full marathons and specializes in sports medicine in Washington D.C. "In general, it's not something that we expect to kill you. It's odd and freaky to have three die in the same part of the course."