CHICAGO – A Michigan man died Sunday while running the Chicago Marathon as stifling heat and smothering humidity forced race organizers to shut down the course midway through the event, authorities said

Chad Schieber, 35, collapsed while running on the city’s South Side and was pronounced dead shortly before 1 p.m. at a Veteran’s Affairs hospital, according to the Cook County Medical Examiner’s office.

An autopsy on the Midland, Mich. man was scheduled for Monday.

“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.

George Chiampas, the race’s medical director, said witnesses have reported seeing Schieber collapse and become unresponsive.

“It sounds like he lost his pulse very fast and died on the race course,” Chiampas said at a news conference after the race.

Meanwhile, more than 300 people were taken to area hospitals because of heat-related ailments.

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

Organizers decided to divert runners from the race route to the starting area shortly before noon as a precautionary measure when temperatures neared 90 degrees. Ailing runners were provided with medical attention and cooling misters.

Organizers closed the second half of the course just before noon, four hours after the annual race started with temperatures already touching 73 degrees with 86 percent humidity.

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.

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 just a brutally hot day.”

Lori Kaufman, a runner from St. Louis, said the fire department turned on hydrants to hose people down along the course.

Associated Press freelancer Benjamin Sylvan contributed to this report