WALNUT CREEK (KRON) — A 21-year-old soldier from Walnut Creek died after falling ill on his first day of U.S. Army Ranger School in Georgia.

2nd Lt. Michael Parros died after being treated for hyponatremia, an illness caused by unusually low sodium levels in the blood that can result from drinking too much water, according to Army officials at Fort Benning, where he was in training.

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

Parros had reported to Fort Benning for training on June 27 after graduating from the U.S. Army Military Academy at West Point in May.

Parros had completed a training session on hand-to-hand combat and gone to dinner before he became ill on Monday, Tongret said. Parros fell ill on Monday and died on Wednesday at the hospital.

The Army is investigating, which is standard practice when a soldier dies, Tongret said.

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.

Parros was a former football player at De La Salle High School in Concord, he played multiple positions for De La Salle’s football team, which won the state title in December 2011, in his senior season before he graduated in 2012.

Parros went on to play football, hockey, soccer as well as being a member of the Cultural Affairs Club at the U.S. Army Military Academy.

“This is a tragic loss,” Lt. Co. Matthew Weber, who oversees the Infantry Basic Officer Leadership Course on Fort Benning, said. “While 2nd Lt. Parros was only with us for a short time, he showed so much potential and was the epitome of the kind of soldier you want to serve with. We are truly saddened to lose a member of our Army family.”

“Our thoughts and prayers are with the Parros family and friends during this exceptionally challenging time,” Weber said.