Officials say there is a growing suspicion of foul play in the deaths of these extremely endangered species whose number has dwindled to about 1,500.
State Tourism, Culture and Environment Minister Masidi Manjun confirmed the latest discovery of decomposing bodies of a male and two female elephants in the Gunung Rara Forest Reserve where 10 others were found dead since December 29.
There is a possibility that more may be found as wildlife officials believe that all the 13 elephants and a rescued calf were part of a single herd.
They are believed to have consumed some form of natural or pesticide poison while roaming in the Yayasan Sabah forest management that is locked between Sabah's Maliau Basin and Danum Valley forest reserves.
Masidi said that there was growing suspicion of foul play in their deaths.
"No amount of laws and enforcement can be effective if people do not have the sense of responsibility that these animals belong to the people," he said yesterday.
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He has ordered his ministry's permanent secretary to take over as chairman of the special task force set up to investigate the deaths, which have been dubbed a "conservationist nightmare".
Meanwhile, the Sabah Wildlife Department is waiting for a chemical report on the possible causes of the elephant deaths.
Sabah Chief Minister Musa Aman said the deaths of the elephants were "unacceptable".
"I want no stone left unturned in this matter. The culprits must be brought to justice," he said in a statement.
Musa said the state government viewed the matter seriously as Borneo pygmy elephants are an endangered species and were part of Sabah's rich and exotic wildlife.
"People come from all over the world just to catch a glimpse of these animals. It is unfortunate that there are people who would resort to such a horrific act," he added.
Only about 1,200 Borneo pygmy elephants are estimated to be left in the wild.