CHICAGO (CBS) – An autopsy on Monday was inconclusive in determining why a 35-year-old firefighter from Greensboro, N.C., collapsed and died while running the Chicago Marathon.

William Caviness, a captain with Greensboro Fire Department, collapsed about 500 yards from the finish of the marathon at about 10:30 a.m. on Sunday.

Marathon spokeswoman Robin Monsky said five emergency doctors working for the marathon attended to Caviness right away when he collapsed and put him in an ambulance. Doctors who attended to Caviness at the scene got a pulse, but he died an hour and 45 minutes later at Mercy Hospital and Medical Center.

An autopsy on Monday was inconclusive, pending the results of toxicology tests, which could take four to six weeks, according to the Cook County Medical Examiner’s office.

The weather was warm on Sunday, with temperatures in the 70s, but marathon officials said fewer runners sought medical treatment than in past years and that the race course had plenty of fluids for the runners.

Caviness was running the marathon with his brother, Sean, and was raising money for burn victims through the International Association of Fire Fighters’ Burn Foundation. According to a website for Caviness’ fundraising efforts, he had raised $7,651.

Caviness’ wife was in Chicago with him for the race. Fellow firefighters said he was a big runner and Greensboro Deputy Fire Chief Clarence Hunter said he was an up and coming leader in the department. He had been a firefighter for eight years

The last death at the marathon was in 2007, when the marathon was halted after a runner died and at least 49 others were hospitalized due to heat-related illnesses amid record-setting heat and humidity.