A 21-year-old Army officer died after falling ill during his first day at the military‘s grueling Ranger School in Georgia.

The soldier, 2nd Lt. Michael R. Parros, died Wednesday at a hospital two days after he became sick during training at Fort Benning. The Army said in a news release that doctors treated Parros for hyponatremia, an illness caused by unusually low sodium levels in the blood that can result from drinking too much water.

“Typically it’s over-hydration, but we don’t really know what happened in this case,” John Tongret, a Fort Benning spokesman, said Thursday.

The two-month Ranger School course tests soldiers’ abilities to overcome fatigue, hunger and stress during combat operations. The latest class kicked off on a sweltering Monday when temperatures at Fort Benning, located outside Columbus, reached 95 degrees and the heat index hit 103 degrees, according to the National Weather Service.