A former Thai Navy officer died on Friday as he ran out of oxygen while assisting the rescue of 12 boys and their football coach, who have been trapped in a cave in Thailand’s northern province of Chiang Rai for nearly two weeks. “A former navy SEAL who volunteered to help died last night about 2 am [12.30 am Indian Standard Time],” Chiang Rai Deputy Governor Passakorn Boonyaluck told reporters, according to AFP.

The former officer, identified as Saman Kunan, 38, was on his way back from delivering an air tank to the group when he lost consciousness. Rear Admiral Arpakorn Yookongkaew, the Thai navy SEAL commander, said attempts to revive Kunan were unsuccessful.

The level of oxygen in the cave has dropped to 15% from the normal atmospheric level of 21%, local authorities told The Guardian. Narongsak Osatanakorn, the governor of Chiang Rai province, said that the presence of hundreds of rescue workers inside the cave has depleted the oxygen supply inside it. Around 110 Thai Navy SEAL divers are participating in the rescue mission, along with other divers from the United States, Britain, China, Australia and some other countries.

It is believed that the group entered the Tham Luang cave in the northern province of Chiang Rai when it was dry. Sudden heavy rains may have blocked the exit. On Monday, the Thailand Army said the group will either need to learn to dive or wait months for flooding caused by the monsoon to recede before they can be rescued.

The boys are believed to be trapped between 800 metres and 1 km below the surface. Thanes Weerasin, the president of the Engineering Institute of Thailand, who is overseeing the drilling crew in a forest above, said he was exploring one tunnel with a large hole – around 1.2 metres by 1 metre – into which the rescuers could drill to rescue the group.