Samarn Poonan, a former member of Thailand's elite navy Seal unit who was part of the rescue team in Chiang Rai, died on Thursday night after entering the cave to lay oxygen tanks along a potential exit route, the Seal commander said.
"The conditions in the cave are tough," Admiral Apakorn Yuukongkaew, commander of the Seals unit, told reporters on Friday.
"Once he placed the oxygen tanks he became unconscious on his way back. His buddy tried to administer first aid, when there was no response he tried to move him," Apakorn said.
"We won't let his life be in vain. We will carry on".
Volunteers at the Tham Luang cave in northern Chiang Rai province were shaken by Samarn's death.
"A navy Seal just passed away last night. How about a 12-year-old boy that will have to pass through?" Rafael Aroush, an Israeli living in Thailand and volunteer who arrived at the cave site on Thursday, told Reuters.
"There will be rain and many things could go wrong. I don't want to say it, but it could be a catastrophe," he said.
Chiang Rai Governor Narongsak Osottanakorn said on Thursday that rescuers were preparing a five km (three miles) "oxygen pipeline" as part of preparations for the group's extraction but added that the boys would not be coming out soon.
"You see we are increasing the number of people going inside the cave. So we have to fill it up with oxygen," Narongsak told reporters.
Rescuers, including international teams, are considering alternative ways to bring the group out before heavy rains hit the country's north next week which could further hamper the rescue operation.
"We have considered many alternatives to find the most feasible ones," Narongsak said.
Deep water
Rescue alternatives include teaching the boys to dive and then swim out, a highly risky venture, remaining in the cave for months until the wet season ends and flood waters recede, or drilling a shaft into the cave from the forest above.
The boys, aged between 11 and 16, and their assistant coach were found inside the cave on Monday, after nine days underground, hungry but in good spirits. They went missing after they set out to explore the cave on June 23.
Rescuers are deciding how to remove the group but have been slowed down by logistical issues including high water levels inside the cave and narrow, flooded passages which would require the boys to dive alone.
The Thai navy is teaching the boys the basics of diving, with a view to guiding them out through flood waters.
But getting them out won't be easy. The boys will have to be taught how to use scuba diving gear and how to navigate a cave that has frustrated even the most expert divers.
"Regarding the plan for the 13 to swim or dive, there is only one critical point which it is risky: It is where every boy has to dive alone. The point is very narrow ... It is very deep water. The distance is pretty long," said Narongsak.
Some of the boys cannot swim.
But rescuers are considering other options including keeping the 13 inside the Tham Luang cave until the flood waters recede, at the end of the rainy season in about four months.
Others say the boys could be out in days if the weather is on their side and enough water can be pumped out of the cave to enable the boys to get out the same way they got in, on foot, perhaps with some swimming.
Another possibility would be to find an alternative way into their chamber, such as drilling a shaft into the cave from the forested mountain above.
Heavy monsoon rains are forecast for next week in most of the north, according to Thailand's meteorological department.
