Three critically-endangered Sumatran elephants found dead in Indonesia\'s Riau province were probably poisoned in a revenge attack by palm oil plantation workers, an official said Monday. The decomposing carcases of the female elephants, including a year-old baby, were found in a jungly area just outside the Tesso Nilo National Park in Riau province on Saturday, park chief Kupin Simbolon told AFP. \"We believe they died a week earlier, from suspected poisoning,\" he said. \"The elephants probably entered a palm oil plantation and destroyed the huts belonging to workers, who then sought revenge by poisoning them,\" he added. At least 17 elephants had died this year at the park and surrounding districts, mostly from suspected poisoning, Simbolon said. \"The elephants are protected animals. We must not let this happen again. We are working with the police to hunt down the perpetrators in this latest case,\" he added. Indonesia is the world\'s biggest palm oil producer and growing demand has put pressure on the nation\'s already threatened tropical rain forests. There are fewer than 3,000 Sumatran elephants remaining in the wild, according to the International Union for the Conservation of Nature, a 50 percent drop in numbers since 1985. International conservation group WWF changed the Sumatran elephant\'s status from \"endangered\" to \"critically endangered\" in January, largely due to severe habitat loss driven by palm oil and other plantations.