10:10 a.m. ET, July 10, 2018

He died while trying to rescue the boys — and made teams rethink the mission

Former Sgt. Saman Kunan, an ex-SEAL, died on July 6 while working on the mission to free the trapped boys.

The 38-year-old, who was a triathlete , ran out of air while underwater as he returned from delivering oxygen tanks to the cavern where the boys were located.

Kunan's death hammered home the difficulty of bringing out the group — and may have changed the way rescue teams approached the operation.

Finnish volunteer diver Mikko Paasi, a long-term resident of Thailand, talked to CNN while the rescue mission was still in motion. He said Kunan's death had changed the mood on the ground and made real for rescuers just how dangerous the mission had become.

One of Kunan's longtime friends, Sgt. Anuram Kaewchano, told CNN he was shocked to learn the news.

"I can't believe this happened," he told CNN by phone. "He was very fit, he exercised every day, and he was a triathlete. Our last trip together was to Malaysia."