Archaeologists excavated the site of a previously unknown 15th-century mass sacrifice in Peru and discovered the skeletons of 137 children, three adults and more than 200 juvenile llamas, according to a new study. The researchers say that the displaced ribs and cut marks on the sternums of the human and animal bones suggest their hearts were removed.

The findings were published in the journal PLOS on Wednesday.

The site was excavated between 2011 and 2016 after local residents said they saw bones in eroding roadside dunes.

The well-preserved remains were found over 700 square meters where the Chimú state culture lived along Peru’s coast during the 15th century. The Chimú state was at its peak then, one of the most powerful in the Americas.

From A.D. 900 to 1500, the fight for political, economic and religious control of the region caused warfare and massacres across Peru’s northern coast as the state expanded. This was also when the Chimú state flourished, controlling hundreds of miles of coastline and far inland in multiple directions. Its people were at the heart of an extensive trade network and used hydraulic canals to support their prosperous agriculture.

At the heart of it all was the capital, Chan Chan, one of the largest settlements at the time. It included palaces, gardens, temples and plazas. It was perfectly situated between the fields, desert, mountains, wetlands and the Pacific Ocean.

While sacrifices were part of ancient cultures, there was previously very little evidence that they happened along Peru’s northern coast. A site in a Peruvian seaside town excavated in 1969 revealed 17 children and 20 llamas buried together. It was presumed to be a sacrificial site, but at the time, it was an isolated finding.

The newly excavated Huanchaquito-Las Llamas site is beach sand about a quarter-mile from the shore, and part of the site has been disturbed in recent years due to construction.

At this site, there were no grave goods to speak of, and the bodies were found in unusual positions, suggesting that it wasn’t a typical burial ground.

The children were boys and girls between the ages of 5 and 14, with the majority falling between 8 and 12. Anatomical and genetic evidence, including skulls that had been modified and shaped while the children were growing, showed that they came from different ethnic groups and regions.

The researchers said there would be even more children among the total if incomplete remains from the site had been included.

The children were buried in groups of three, increasing in age. Some of them wore face paint or headdresses that were applied before the sacrifice. The llamas were next to or on top of the human skeletons and were alternating in color, largely brown and beige.

The children were buried facing the sea, and the llamas were facing the mountains. The children wore no shoes, only simple cotton shrouds, and were found curled on their sides, lying flat or crouched on their backs.

The study authors believe that someone with “an experienced hand” sliced open the sternums of the children and the llamas and forcefully opened their chests to remove their hearts.

There were also three adults: one man and two women. One woman was 18 years old, and the other was between 20 and 30. One probably died from a blow to the head, and the other showed signs of blunt force trauma to the face, but there was no identifiable cause of death. The man was between 30 and 40 years old and had multiple rib fractures, and no cause of death could be identified in his case, either.

The two women were buried face-down and crouched on their knees, and the man was buried on his back. Due to their placement and proximity to the children, they were associated with the ritual sacrifice.

The llamas were all less than a year and a half old, with the majority being less than 9 months old. The researchers believe this was purposeful, to match with the young ages of the children. While there were a range of different colors of llamas, including beige, light brown, dark brown and mixed colors, brown was the most common, which suggests that it was chosen on purpose.

To the north, a single llama was found buried with a pair of ceramic jars and wooden paddles, all associated with the Chimú funerary process.

A thick layer of mud covered the area beneath the skeletons, suggesting that a massive rainstorm or flood happened before the sacrifice and may have even inspired it.

Some of the mud was still well-preserved enough to show the footprints of children and llamas as they walked to the site of their death.

Radiocarbon dating allowed the researchers to determine that the sacrifice occurred in 1450. The discovery is the largest known mass sacrifice of both children and llamas in the Americas.

But why did it happen? Only further study will help the researchers learn more about the lives lost at the site. But they believe that the rainfall or flooding from El Niño could have affected the Chimú state’s economic, political and ideological stability.

“This archaeological discovery was a surprise to all of us — we had not seen anything like this before, and there was no suggestion from ethnohistoric sources or historic accounts of child or camelid sacrifices being made on such a scale in northern coastal Peru,” John Verano, study author and professor of anthropology at Tulane University, said in a statement. “We were fortunate to be able to completely excavate the site and to have a multidisciplinary field and laboratory team to do the excavation and preliminary analysis of the material. This site opens a new chapter on the practice of child sacrifice in the ancient world.”

The researchers will continue their investigation to study this connection between child sacrificial practices in the Americas in relation to environmental catastrophes.

“Thanks to National Geographic and FONDECYT Peru we are digging a new site located a couple of miles north of Huanchaquito Las Llamas that has evidence of another massive sacrificial ground,” Gabriel Prieto, study author and assistant professor in archaeology at the National University of Trujillo, Peru, wrote in an email. “Our efforts are concentrated there for the next years.”