Forty-three of the 56 U.S. Army soldiers participating in a physical training exercise last Friday near Fort Bragg, NC, suffered from heat-related illness, according to Reuters news agency.

Of those 43 soldiers, 13 spent at least one night at Womack Army Medical Center. Three soldiers, including one reported to be in critical condition, are still hospitalized.

The soldiers were vying to earn an Expert Field Medical Badge (EFMB), which is considered “the utmost challenge to the professional competence and physical endurance of the Soldier medic,” according to a release that the Fort Bragg Public Affairs Office issued to the media prior to the training exercise.

The physical fitness portion of the challenge includes a 12-mile march to be completed within three hours. Soldiers are required to participate in simulated field dress, which includes wearing a helmet, weapons and a 35-pound rucksack. It was during this march that the illnesses occurred.

One of the officers supervising the march said that the temperature during the early-morning training exercise ranged from 70 to 76 degrees but that the heat index was 95 degrees when the soldiers started the march.

“The event that happened this morning was a culmination of two weeks of intensive training, combined with stress,” Colonel Scott Putzier said.