An autopsy showed a heart condition, not record-setting heat, killed a Michigan police officer who died during an unusually hot and humid Chicago Marathon.

Chad Schieber, who collapsed while running on the city's South Side, had a mitral valve prolapse and did not die from the heat, the medical examiner's office said Monday.

The 35-year-old Schieber, from Midland, Mich., was pronounced dead shortly before 1 p.m. Sunday at a hospital.

Several other people collapsed, and at least two remained in critical condition Monday, as record heat and smothering humidity forced race organizers to shut down the course midway through the event.

"Obviously very sad news, and our thoughts and prayers are with the individual's family," said Shawn Platt, senior vice president of LaSalle Bank, the marathon's sponsor.

Schieber was a 12-year police veteran in Midland, a city of about 42,000 in central Lower Michigan. He worked as a field training officer and community relations officer and implemented the department's child DNA identification program, the Midland Daily News reported.

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.

Also 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 200 yards from the finish line. Medics 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 3 1/2 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."

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."