Officials believe the deaths of three elephants, believed to be from the same family, outside the Kaeng Krachan National Park in Prachuap Khiri Khan, were caused by eating agricultural pesticides.
An initial inspection of the pachyderms - one male and two females - found no injuries, except a bite at the tip of the trunk of the youngest elephant.
All of them had blood on their eyes and around their mouths.
Officials believe the three jumbos ate contaminated crops from a villagers' farmland.
They might have also attempted to drink water from the village pond, Kamon Nuanyai, chief of Kaeng Krachan National Park, said Wednesday after the inspection.
Local villagers spotted the three elephants near the pond in the compound of the Asom Burapa meditation centre in Ban Chaloemkiat Pattana in Huai Sat Yai sub-district of Hua Hin district early Wednesday morning.
The bodies of the elephants were found lying in a pineapple plantation, surrounded by rubber trees in an area which is roughly two kilometres away from Kaeng Krachan National Park.
The Prachuap Khiri Khan governor, park officials, Nong Phlap police officers, border patrol police and environmental advocates inspected the scene.
The dead pachyderms were a male elephant aged about 10 years with 30-centimetre-long tusks, a female aged about five years old, and a female baby elephant aged about two.
Mr Kamon believes they were from a herd of 15 elephants seen by locals foraging for food near the park.
The elephants died on Tuesday, he said, adding veterinarians are trying to determine what exactly killed them. Last night police took a plantation worker in for questioning in connection with the deaths.