The diver, whose mission was to bring oxygen tanks and carry lines along the passageways, lost consciousness on his way back.
Mae Sai: A former Thai military diver has drowned while helping in the operation to rescue 12 boys and their football coach trapped inside a flooded cave, officials said Friday, highlighting fears over the painstaking operation to extract the youngsters.
His death raises serious doubts over the safety of attempting to bring the group out the same way, through cramped passageways deep inside the waterlogged cave.
Many of the boys, aged between 11 to 16, are unable to swim and none has diving experience.
"A former Seal who volunteered to help died last night about 2am," Chiang Rai deputy governor Passakorn Boonyaluck told reporters at the site, calling it "sad news".
The diver, identified as Saman Kunont, was coming back from a location inside the Tham Luang cave where the kids and their football coach were found on Monday.
"On his way back he lost consciousness," said Thai Seal commander Apakorn Yookongkaew, adding that his friend tried to help bring him out. "We lost one man, but we still have faith to carry out our work."
Asked how the boys could make it out safely if an experienced diver could not, Apakorn said they would take more precautions with the children.
The diver's mission was to bring oxygen tanks and carry lines along the passageways between a forward operating base and the muddy embankment where the children and coach are sheltering.
Even for expert divers the journey is an exhausting 11-hour round trip.
"It's very risky (diving out). Think about it, a Navy Seal just passed away last night, so how about a 12-year-old kid," said Rafael Aroush, an Israeli diver helping the rescue bid.
Officials vowed to investigate the death, and have sent his body to a local hospital for an autopsy.
The accident marks the first major setback for the gargantuan effort, which started almost two weeks ago after the "Wild Boars" team went into the cave in northern Thailand following football practice.
Their ordeal has gripped Thailand, with the nation holding its breath for their safe extraction.
Messages of support and prayers for the team have flooded social media, while the global media has massed at the cave entrance.
But the unprecedented rescue mission has been dogged by the threat of rains.
On Friday morning forecast downpours held off, giving a glimmer of respite to the rescue workers who are pumping out the water, whilst plotting the complicated extraction plan.
The team have been receiving basic training in breathing and managing diving equiptment in case the order to evacuate is given.
But authorities are reluctant to allow them to 'dive out' of the cave, given the extreme risks.
Other options under consideration are waiting out the monsoon, which could take months, or climbing out through a crack in the rock face, if one can be found.