Three other elephants were found poisoned in Matusadona National Park in western Zimbabwe.
The Zimbabwe Parks and Wildlife Management Authority and Zimbabwean police told CNN they have arrested five suspects.
A source in the national parks office says the elephants in Matusadona were poisoned deliberately, probably with orange peels laced with cyanide. Those in Hwange were poisoned using a salt lick spiked with the same poison.
While four elephants were found with their tusks removed, others were found with the tusks attached, leading rangers to believe the poachers had been disrupted before the tusks could be removed, the parks office said.
Several of the poached elephants were too young to have tusks; the youngest was 3 months old, the office said.
"It is a horrible way to die," Carel Verhoef, a longtime professional African guide, told CNN from Cape Town, South Africa.
Cyanide affects breathing and functionality, and elephants ultimately die of suffocation.
"If the salt licks were poisoned, then other animals would have died, so it would affect the whole ecosystem," he said.
Vultures and impalas were also found dead at the scene, also poisoned by cyanide, the parks office source said.
This is not the first time that poachers have used cyanide. In 2013, around 300 elephants were poisoned in Hwange National Park in a single incident, the victims of poachers fulfilling a global demand for illegally trafficked ivory.
Cyanide is relatively easy to obtain in Zimbabwe because of the number of mines in the country, and conservationists say poachers prefer it because it is quiet, as gunshots would draw attention from parks officials.
But for elephants, known to be family-oriented animals, poaching has implications far beyond the physical trauma.
"It would be extremely disruptive to the herd and its structure," Verhoef said of the incident. "If these elephants were part of a breeding herd and the adult cows died, then the sub-adults would not have any structure and the chances are high that they will become 'problem' elephants."
Elephants have long been recognized for their intelligence.
"They reason and think and are identical to us," Dame Daphne Sheldrick, of the David Sheldrick Wildlife Trust, told CNN from her home in Kenya in July. "They mourn and grieve just like us. So they understand that poaching is a growing problem, they see death and destruction every day."
There are around 400,000 elephants remaining in Africa. And across the world, an elephant dies every 15 minutes, Sheldrick said.