The carcases of at least 40 poisoned elephants have been discovered in Zimbabwe's national parks since last week, renewing concerns about poaching on protected lands.On Tuesday, rangers at Hwange National Park found 26 elephants who died of cyanide poisoning, The Associated Press reported. The troubling discovery comes after the bodies of 14 others, also apparently dosed with cyanide, were recovered last week."Cyanide poisoning is becoming a huge problem here and we are struggling to contain it," conservationist Trevor Lane, founder of Bhejani Trust, told the AP.Although no arrests have been made, rangers plan to increase their patrols in light of the slayings, the AP reported. Some tusks were left behind after the recent killings, which officials said is a sign that the patrols are disrupting poachers' work.At least one Zimbabwean official blamed a U.S. ban on elephant hunting for the incidents, saying the policy is increasing demand for poaching, according to the AP.Poaching remains a serious threat to the world's elephant population across Africa, despite efforts to curb the illegal ivory trade. Last month, a Kenyan conservation center reported that an elephant had returned to the sanctuary after being hit by darts. That elephant made a full recovery, but more than 20,000 African elephants were poached in 2013, according to one estimate.

Hwange National Park, the country's largest game reserve , made headlines earlier this year when the slaying of a lion named Cecil just outside the park sparked international outrage. Officials there announced this week that the U.S. hunter who killed the lion acted within the law and will not face charges.