FORT BENNING, Ga. (AP) — A 21-year-old Army officer has died after falling ill during his first day at the military's grueling Ranger School in Georgia.

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

Fort Benning spokesman John Tongret said Thursday that Parros of Walnut Creek, California, had finished a hand-to-hand combat training session and went to dinner before he became ill. The Army is investigating, as it does whenever a soldier dies.

The two-month Ranger School course tests soldiers' abilities to overcome fatigue, hunger and stress during combat operations.