Thai navy SEAL commander Arpakorn Yookongkaew said the volunteer diver, Saman Kunan, had been placing compressed air tanks along an exit route to assist the boys in their escape from the cave. "He did not have enough on his way back", said an official.
The warning came a day after a Thai diver died during part of the rescue operation, marking a deadly turn in what started out as a celebration of one of the boys' birthdays at the Tham Luang cave complex in northern Chiang Rai province. It is tied to a telephone line to provide a channel of communication for the kids, who are stuck deep in the complex but are being looked after by four SEALs, including a medic.
Kunan's death underlines the danger the boys, aged 11 to 16, and their 25-year-old coach are in.
British divers Rick Stanton, a fireman in his fifties from Coventry, and John Volanthen, an IT consultant based in Bristol in his forties, were the first to reach the group.
Authorities have been racing to pump out water from the cave before more storms hit the region in the coming days and send water levels rising again.
But the path to freedom is hugely risky for the boys, some of whom are unable to swim and have no scuba experience in an environment with low visibility that confounds even the most skilled.
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PO Gunan's death has underscored the dangers behind the search efforts.
The strategically placed canisters allow divers to stay under water for longer during what is about a five-hour trip to reach the stranded youngsters and their coach.
Attempts to resuscitate him failed.
Food, electrolyte drinks, drinking water, medicine and oxygen canisters have been delivered to them.
"Saman left us while working with his SEAL team brothers at a time where all frogmen put their arms around each other and move forward to complete the mission that is set".
Chiang Rai governor Narongsak Osottanakorn said all members of the group did not have to be brought out at the same time, but could be extracted over days, depending on the weather. They've practiced wearing diving masks and breathing, in preparation for the diving possibility. Rescue crews have assessed that the ongoing effort to pump water out from the main entrance of the cave has reached a point where it's becoming less and less effective, and placing pumps further inside the cave - beyond the 800 meter mark, where they now are - does not seem possible.
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The navy is teaching the boys the basics of diving, with a view to guiding them out through flood waters. The idea is to get some headroom so the boys would not be reliant on scuba apparatus for a long stretch and could keep their heads above water.
He said he wanted to reduce risks but added that falling oxygen levels inside the cave were another "really big concern".
His engineers are working with the army, exploring an area they believe to be just hundreds of metres from where the boys and their coach are trapped.
"We are awed by the bravery and strength that these young boys and their coach have shown amidst such frightening circumstances", it said on its website.
One of his enterprises is Boring Co., which digs tunnels for advanced transport systems and has advanced ground-penetrating radar.
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