Anthropologists in Mexico say the remains of 167 bodies found in a cave in the southern state of Chiapas were part of an ancient burial ground.
The National Anthropology Institute said tests showed the remains dated back to the eighth century.
Scientists hope pottery found in the cave will help them determine the community those buried belonged to.
It was first feared the bodies could belong to victims of the decades-long civil war in neighbouring Guatemala.
Farmers had found the bodies in a cave on the Nuevo Ojo de Agua ranch, some 20km (11 miles) from the Guatemalan border, and alerted the authorities.
Initial tests suggested the bodies were at least 50 years old, leading some activists in Guatemala to speculate they may have belonged to victims of the 1960-1996 civil conflict.
But forensic experts have since said the skulls showed signs of a deformation typical of native communities dating back 1,000 years and more.
The Maya people who thrived in the region for nearly 2,000 years used planks to flatten and elongate the skulls of their children.
Anthropologists continue to examine the remains in an effort to determine the sex, age and ethnic make-up of the bodies.