Insurgents in India are suspected of using AK-47 assault rifles to hunt rare, one-horned rhinos.
Poaching has been rampant in and around the Kaziranga National Park in Assam state, where 21 rhinos were killed last year and eight so far this year.
Six of them were killed in the past month alone using AK-47s, leading police to believe insurgents have joined the threat posed by traditional poachers, Assam state police chief Jayanta Narayan Choudhury said.
"This is serious," state politician Tarun Gogoi said, adding that he was asking federal investigators to launch a probe into the rhino killings.
Assam, in the northeast of India, has struggled with insurgents.
An estimated 2,500 out of the world's 3,000 one-horned rhinos live in Kaziranga.
Powdered versions of their horns are coveted in many Asian countries as a medicine or an aphrodisiac.
On Tuesday, a group of protesters gathered at the park and stripped to their underwear to express anger over the rhinos' killings and to demand more state government action to protect the rare animals.
Some foreign tourists also lent their support to the protest.
"I had come here to see the rhinos, but when I heard the animals were being killed by poachers at frightening regularity, I decided to join this protest," Kenyan tourist Jenny Turner said.
The World Wildlife Fund said the Assam state's porous borders with neighbouring Bangladesh and Myanmar means that such weapons are easily available there and that poachers have easy access to illegal wildlife trade networks.
The 185-square mile Kaziranga National Park is about 155 miles east of Gauhati, the state capital.
Nearly two dozen rebel groups in the northeast have been fighting for independence or wide autonomy for decades and accuse India's central government of exploiting the region's rich natural resources.