Runners start the Philadelphia Marathon on Sunday. (Joseph Kaczmarek / AP)

Jeffrey Lee, 21, died after crossing the finish line in the half-marathon. A California native, he was a senior at the University of Pennsylvania, pursuing a dual degree from the Wharton School and the School of Nursing. He was pronounced dead at Hahnemann University Hospital.

G. Chris Gleason, of Clifton Park, N.Y., collapsed about a quarter-mile from the finish line. Gleason, 40, was pronounced dead of an apparent heart attack, race officials said, at the same hospital.

Test results on the cause of the deaths will not be available for several weeks.

The marathon course is flat and the race is open to anyone over 16, with no qualifying times required. At least 10 others were rushed to the hospital by ambulance, according to CBS News, but the deaths were the first in the race in 13 years. About 25,000 runners participated, with 10 requiring hospitalization or treatment.

Although both men appeared to have died of heart attacks, according to a police spokeswoman, a Temple University cardiologist told that was unlikely, particularly with a 21-year-old victim. Alfred Bove, who has run marathons, said runners should be checked for undetected heart problems, which can worsen as blood-sugar, potassium, and sodium levels drop during a race.

“I keep telling people, if they're a runner, they should, in fact, get a checkup and make sure they understand their risks,” Bove said.