A former Thai navy SEAL has died during the operation to rescue 12 boys and their football coach trapped in a flooded cave, the authorities have said.
The former elite diver died while working as a volunteer to place oxygen tanks along a potential escape route for the boys. It comes as the rescue team face a race against time to free the boys before a new rainstorm arrives.
Speaking at a news conference on Friday morning, the Thai navy SEAL commander in charge of the rescue effort named the veteran diver as Samarn Poonan. Samarn died during the overnight mission after he fell unconscious from lack of oxygen, the commander said.
At the same time, officials also raised concerns on Friday that oxygen levels in the cave are running low, with so many people now working in them to supply the boys and prepare for their rescue. It shows how, four days after they were discovered alive, the boys still face many obstacles before they can be brought to safety.
"A former SEAL who volunteered to help died last night around 2am," Chiang Rai's deputy governor, Passakorn Boonyaluck, told reporters at the rescue site. "His job was to deliver oxygen. He did not have enough on his way back."
The diver was 38 years old and had left the SEALs, Thailand's elite naval unit, but volunteered to return after seeing the reports of the missing youth football team that have dominated the news in Thailand for nearly a fortnight.
The boys, aged 11 to 16, have been trapped for almost 13 days since they went to explore the caves after a football match on 23 June. They were discovered by two British volunteer divers on Monday night, but authorities are still trying to establish the safest way to get them out of the flooded cave network.
The safest option could be to simply wait until the caves dry out naturally - but that could take months, with Thailand's monsoon season not ending until around October, and there are no guarantees that more rain wouldn't fill up the chamber where the boys are trapped.
So authorities are working towards a plan where the boys are helped to dive out of the chamber along the same route rescuers used to find them. That journey is currently taking experienced divers about five hours, while the boys themselves have no previous experience using diving equipment and some reportedly cannot even swim.
At the news conference, a spokesman was asked how children with no experience could be ferried out alive when an adult former navy SEAL had died. The spokesman replied: "We have to try everything," according to the Guardian.
"The conditions in the cave are tough," Admiral Apakorn Yuukongkaew, commander of the SEALS unit, told reporters.
"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".
Last night, officials said they were worried the amount of oxygen in the chamber where the boys are trapped is depleting. Seven navy SEALs, including two medics, have been looking after the group in the chamber, and others have been arriving periodically to bring supplies.
Chiang Rai governor Narongsak Osottanakorn said rescuers would try to feed a 5km (3-mile) "oxygen pipeline" into the chamber as part of preparations for the group's extraction.
"You see we are increasing the number of people going inside the cave. So we have to fill it up with oxygen," Narongsak said.
Volunteers at the Tham Luang cave in were shaken by the turn of events on Friday morning.
"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 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.
