The Cook County medical examiner determined Monday that Chad Schreiber, 35, a Midland, Mich., police officer, suffered from a mitral valve prolapse that caused him to collapse and die 19 miles into Sunday's grueling 26.2 mile race.

With temperatures at a record 88 degrees, the marathon had to be halted as hundreds of runners began suffering from the heat and dehydration. The Chicago Tribune said more than 315 runners required medical treatment.

Mitral valve prolapse is a condition in which a heart valve doesn’t close properly and is not related to heat.

Schreiber and his wife had three children and he was named police officer of the year in Midland this year.