updated 1:43 p.m. ET Oct. 8, 2007

CHICAGO - Two runners died Sunday during separate, unrelated races. A Michigan police officer died and dozens of others needed medical care while running the Chicago Marathon as record heat and smothering humidity forced race organizers to shut down the course midway through the event.

But it was a heart condition, not the heat, that killed Chad Schieber, an autopsy showed Monday.

Schieber, 35, collapsed well into Sunday’s race, leading to speculation his death was related to the 88-degree temperatures that forced organizers to announce they were halting the race. Several other people collapsed, and at least two remained in critical condition Monday.

The medical examiner’s office said Schieber had mitral valve prolapse and did not die from the heat, according to the Cook County Medical Examiner’s office.

The condition is a “common heart disorder” in which the valve between the “heart’s left upper chamber and lower left chamber does not close properly,” according to MayoClinic.com. In most people, mitral valve prolapse is harmless and doesn't require treatment or changes in lifestyle. It also doesn't shorten your life expectancy. However, in some people the progression of the disease requires treatment, the Web site says.

Schieber, a Midland, Mich., police officer, was pronounced dead shortly before 1 p.m. at a Veteran’s Affairs hospital, according to the medical examiner’s office.

“It sounds like he lost his pulse very fast and died on the race course,” George Chiampas, the race’s medical director, said at a news conference. Chiampas said witnesses reported seeing Schieber collapse and become unresponsive.

At least 49 people were taken to hospitals, while another 250 were treated onsite, many for heat-related ailments. Chicago Fire Department officials said they used 30 ambulances from area suburbs. Three people were in critical condition at Northwestern Memorial Hospital late Sunday, according to spokeswoman Jennifer Monasteri.

Also on Sunday, an unidentified runner died during the Army Ten-Miler race in Arlington, Va., near the finish line at the Pentagon. The runner collapsed about about 200 yards from the finish line. Medics responded immediately and took the runner to George Washington University Hospital. The cause of death was not known.

About 10,000 of the 45,000 registered runners never even showed up for the 30th annual Chicago race, while another 10,934 started but didn’t finish, officials said.

The high heat index prompted organizers to stop the race at 11:30 a.m., about 3½ hours into the run. Runners who hadn’t reached the halfway point were diverted to the start and finish area, while those on the second half of the course were advised to drop out, walk or board cooling buses, Platt said.

Race director Carey Pinkowski said organizers were concerned that emergency medical personnel wouldn’t be able to keep up with heat-related injuries as the weather turned more cruel.

“We were seeing a high rate of people that were struggling,” Pinkowski said. “If you were out there at 1 o’clock, it was a hot sun. It was like a summer day, it was just a brutally hot day.”

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Kenya’s Patrick Ivuti won, leaning at the finish line to edge Jaouad Gharib of Morocco by 0.05 seconds. Ivuti, competing in only his second major marathon, was timed in 2 hours, 11 minutes, 11 seconds in the closest finish in the race’s history. He was the fifth straight Kenyan to win the race.

Ethiopia’s Berhane Adere rallied to successfully defend her women’s title, finishing in 2:33:49 after passing a surprised Adriana Pirtea, who had a comfortable 30-second lead after 24.8 miles.

By 10 a.m., temperatures had already reached a race-record of 88 degrees. The previous marathon record of 84 degrees was set in 1979. Pinkowski said it was a tough decision to stop the race, but a prudent one.

Lori Kaufman, a runner from St. Louis, said she was told to start walking by mile No. 14. She said she didn’t have enough water or Gatorade.

“We had a lot of spectators just handing us bottles of water which helped a lot,” Kaufman said. “Every medic station that we passed was full of people. I mean they were not doing well.”

Some kept going and helicopters hovered over the race course while police officers shouted through a bullhorn and warned runners to slow down and walk.

Fire hydrants were opened along the course and some residents who live along the race route used garden hoses to spray water on the weary runners.

Paul Gardiner, a runner from England, said the weather made for a “brutal” run.

“We were at about 18 miles and we heard they canceled it and that kind of sent a little bit of concern through the crowd,” Gardiner said. “It’s just it’s impossible to run.”

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