Amid several sections, Sugiyama has uncovered the remains of sacrificed victims.
Analyses by Spence of the University of Western Ontario suggest the sacrificed victims came from outside Teotihuacán, possibly as captives brought back from distant territories or battles.
The clues come from oxygen isotopes in bones, which act as geological markers. "They tell you where a person was at a particular time," he said.
Climate and altitude are among factors that affect the isotopes. The isotopes found in remains of pyramid victims differ from those unearthed in city homes.
Spence has also found evidence that the health of Teotihuacán's population declined in the city's final century. Residents' teeth have tell-tale lines that form in childhood during episodes of severe stress, such as malnutrition or infection.
"Basically growth stops as the body concentrates on survival and repair," he said. "Then as the stress passes, the growth continues again. But there's a line left in the tooth that represents the stress episode."
Because teeth only grow during childhood, scientists can put a general age to when the stress happened. These signatures of bodily stress remain in adult teeth.
"We have shown that in the last century of the city there is a growing problem of some sort. We get more and more indications showing up in adult teeth," he said.
The largest unanswered questions about Teotihuacán concern its demise. Why, for example, was the city largely abandoned around A.D. 650?
The recent excavations are revealing new bits of information that help piece together an answer.
"We don't know exactly what happened at the final stage, but we know certainly the city was destroyed by man, not by natural disaster," Sugiyama said.
Researchers are uncertain whether insiders or outsiders caused the destruction, Sugiyama said, but they do know that the instrument was fire, particularly on Teotihuacán's monuments.
The archaeologist says an invading army could have set fires to the monuments as a signature of their conquest.
Spence, however, says the evidence suggests to him the fires were set during an internal revolt.
According to his theory, the deteriorating health of the city's poor was likely exacerbated by a drought or a disruption to the food supply. This spurred a revolution against the ruling elite and their symbols of powertemples, pyramids, and palaces.
"The destruction seems to have skipped the vast majority of the city and focused on the elite and punished the elite. That suggests a revolt to me," he said.
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