The majestic pyramids of Teotihuacan were built high in the Valley of Mexico 2000 years ago by a nameless people who dominated Mesoamerica for hundreds of years, only to disappear virtually without a trace between 600 and 700 A.D.

The Teotihuacanos' origin, the structure of their government, the reasons for their demise and even the very nature of their society remain largely a mystery, as tantalizing to modern-day archaeologists as to the Aztecs who inherited their monuments, but never knew who built them.

Archaeologists recently discovered a previously unknown tomb and four skeletons inside Teotihuacan's Pyramid of the Moon, a find that should help dispel some of the fog, buttressing earlier hypotheses that the people of the pyramids had a strong militaristic and hierarchical bent.

But, like earlier discoveries, the dig sponsored by the National Science Foundation and the National Geographic Society is missing an important element: There are no leaders.

The new skeletons, like those from earlier finds, have their hands tied behind their backs. They were captives, servants, stand-ins, soldiers--underlings of some sort, all 15 to 20 years old.

"We have never found evidence of any kind of governor or important person, in contrast to the Mayas or Aztecs, where we know the leader's name, when he lived and when he died," said Saburo Sugiyama, co-leader of the dig with Ruben Cabrera of Mexico's National Institute of Anthropology and History. "We don't know anything because there is no written history."

Indeed, while analysts at Teotihuacan have found "well over 100 symbols that have some kind of standard significance," Arizona State archaeologist George Cowgill said, "we still don't know if they have phonetic value."

In the absence of a written record, researchers are left to feel their way. Sugiyama, an archaeologist at Japan's Aichi Prefectural University and an adjunct professor at Arizona State University, compared the dig to a 1988 excavation he and Cabrera made at the Feathered Serpent Pyramid, at the other end of the broad avenue known as the Street of the Dead.

Both sites are dated around 200 A.D. Both included green obsidian arrowheads and knives, as well as greenstone figurines, nose rings, earrings and "butterfly necklaces."

Sugiyama found 133 skeletons in the Feathered Serpent Pyramid, all with tied hands, but he also found an abandoned tunnel and an open pit deep within the pyramid, suggesting that looters may have emptied a leader's grave.

"The intent" in the Pyramid of the Moon excavation was to substantiate this theory by "finding a royal burial or high-ranking burial" intact, said Rene Millon, a retired University of Rochester archaeologist who mapped Teotihuacan in the 1960s. Once again, it did not happen.

The new dig did, however, unearth the bones of large felines, canines and predatory birds, Millon said, all symbols of military orders in Mesoamerica, and a further indication that Teotihuacan was a highly regimented and hierarchical society.

Last year in an older section of the Pyramid of the Moon, Sugiyama found a tomb with a single male skeleton with bound hands, accompanied by a wooden cage with jaguar bones inside, an indication that the animal had been buried alive.

Still, although jaguars have been associated with Mesoamerican royalty for at least 2,000 years, royalty does not seem to be buried in the pyramid. "One of the [unresolved] issues is whether this was a monarchical or highly centralized leadership with one person," Cowgill said. "Or was it collective? Nobody's suggesting a democracy, but it may have been governed by an oligarchic elite."

Teotihuacan, 30 miles northeast of Mexico City, vies with Peru's Machu Picchu as the most popular pre-Columbian tourist attraction in Latin America. It sprawls over eight square miles, much of it buried beneath farms, highways, a Mexican military base and five modern-day towns. Only 10 percent of this ancient metropolis has been excavated.

The heart of the site is the 50-yard-wide Street of the Dead, with the Pyramid of the Moon at the north end, the 212-foot Pyramid of the Sun halfway down the avenue on the east, and the Ciudadela, or Citadel, which includes the Feathered Serpent Pyramid, on the southeast corner.

The Teotihuacan pyramids, like those throughout the Americas, are made of stones, debris and dirt, faced with stone sculpture. When their creators wanted a new structure, they simply piled dirt atop the old pyramid and refaced it. The pyramids grew like onions.

Sugiyama probed the Pyramid of the Moon through an 85-foot east-west tunnel that cut through seven separate "phases" before reaching the original pyramid along the north-south axis.

The first three phases were "very simple," he said, with the original phase--believed to be the oldest construction at Teotihuacan--dating from the first century A.D. The male skeleton and jaguar cage were found in phase four, dated about 150 A.D.

Once at the center of the pyramid, excavators turned north and dug for more than 260 feet to phase five before unearthing the four skeletons.

Based on the positioning of the various phases, Sugiyama noted a major architectural reorientation between phases four and five, suggesting that Teotihuacan was making its jump from relatively small ceremonial site to carefully laid-out urban center.

Growth came between 200 and 500 A.D. In his survey, Millon identified more than 2,000 groups of dwellings in the site, capable of housing 150,000 to 200,000 people. Archaeologists have identified whole barrios within the city, including a neighborhood that was populated by Zapotec outlanders from southern Oaxaca.

In its heyday, about 300 A.D., Teotihuacan was the world's sixth-largest city, bigger than Rome, and its influence reached deep into the jungles. Millon said Teotihuacanos founded a 400-year dynasty at the classic Mayan city of Copan.

But it all fell apart sometime after 600 A.D. Archaeologists have found evidence of selective burning of public structures and other destruction, and agree that by 750 A.D., Teotihuacan--as a civilization--had ceased to exist. But the ruins survived through the era of the Toltecs to the rise of the Aztecs and beyond. The Aztecs revered the ancient site and named some of the buildings--the Pyramid of the Sun, the Pyramid of the Moon.

And it was the Aztecs who called the city Teotihuacan--the Place of the Gods. No one knew what its inhabitants had called it.

Special correspondent Garance Burke contributed to this report from Mexico City.