MEXICO CITY (AP) ― Mexican archaeologists say they have found an unprecedented human burial in which the skeleton of a young woman is surrounded by piles of 1,789 human bones in Mexico City’s Templo Mayor.

Researchers found the burial about five meters (15 feet) below the surface, next to the remains of what may have been a “sacred tree” at one edge of the plaza, the most sacred site of the Aztec capital.

The National Institute of Anthropology and History said the find was the first of its kind, noting the Aztecs were not known to use mass sacrifice or the reburial of bones as the customary ways to accompany the interment of a member of the ruling class.

University of Florida archaeologist Susan Gillespie, who was not involved in the project, called the find “unprecedented for the Aztec culture.”

She said Tuesday that when the Mayas interred sacrifice victims with royal burials, they were usually found as complete bodies, not jumbles of different bone types as in this case. And, except for special circumstances, the Aztecs, unlike other pre-Hispanic cultures, usually cremated members of the elite during their rule from 1325 to the Spanish conquest in 1521.

“Although the bodies of sacrificial victims have been found in burials of elite persons in Mesoamerica going back to at least the Preclassic period, funerary deposits for Aztec elites have only rarely been encountered,” Gillespie wrote in an email.

The institute said some of the bones showed what may be cut marks to the sternum or vertebrae, places where a ritual heart extraction might leave a mark, but added that it didn’t seem likely the dead were sacrificed on the spot to accompany the burial because their bones were found separated.

The researchers discovered the skulls of seven adults and three children in one pile, long bones like femurs in another grouping, and ribs in another.

Physical anthropologist Perla Ruiz, who was in charge of the dig, said that might suggest the bones were disinterred from previous burials and reburied with the woman. While some pre-Hispanic cultures disinterred bones as part of ancestor worship, it isn’t clear the Aztecs did.

The burial dates to about 1481 to 1486, based on the “stage” of temple buildings at which they were found. The Templo Mayor, like many sites, was rebuilt by successive generations, one stage atop another.