LUQURMATA, BOLIVIA -- U.S. archeologists are unearthing evidence that Bolivia's ancient, enigmatic Tiwanaku culture was a powerful empire that supported tens of thousands of people in its central city and immediate suburbs.

Digs in this peasant village on the swampy edge of Lake Titicaca are turning up an extensive network of terraced, stone-walled houses and courtyards dotted with tombs, some containing elaborately crafted ceramic bowls and figurines dating back more than 1,000 years.

"Almost every time we've put a hole in a two-kilometer-square area, we've hit a house," said Charles (Chip) Stanish, an anthropologist at the University of Chicago. "We not only hit one house, we hit a lot of them.

"It's becoming clear we're looking at massive population sites."

To Bolivians the discovery is not that surprising. For decades they have maintained that the Tiwanakus were once the Americas' finest and most powerful empire, providing the inspiration for the better-known and relatively modern Incas of Peru.

But the digs are destroying a notion long held in international archeological circles that the culture was little more than a loosely knit confederation of fiefdoms struggling to survive in the thin air, harsh climate and infertile soil of the Bolivian "altiplano" (high plains), about 13,000 feet above sea level.

"It's now being considered a powerful, integrated state," said Alan Kolata, anthropologist from the University of Illinois who is codirecting the excavations with Bolivian archeologist Osvaldo Rivera. They are the first major foreign digs in Bolivia since the 1930s.

Emerging shortly before the time of Christ, the Tiwanakus flourished for scores of generations and reached their technological and artistic peak in the years 600 to 800, before the culture collapsed and disappeared some 400 years later. Their influence extended north and west across Lake Titicaca well into Peru, east into the rain forests of the Amazon basin and south in Chile and Argentina.

Their central temple stood at what is now the potato-farming village of Tiwanaku, 10 miles from Luqurmata, which apparently was a vital suburb. Still standing are a haunting collection of massive stone ruins including an accurate calendar and an elaborately carved Gateway to the Sun.

Also left behind, at points all around the lake and even hundreds of miles from it, are a now-disintegrating network of raised fields that provides the basis for arguments that the Tiwanakus had the resources for a concentrated population and wide-ranging political control.

Excavations have shown that the fields, divided by long irrigation canals, were sophisticated and highly productive.