MEXICO CITY (Reuters) - Archeologists have discovered the ruins of an 800-year-old Aztec pyramid in the heart of the Mexican capital that could show the ancient city is at least a century older than previously thought.

Mexican archeologists found the ruins, which are about 36 feet high, in the central Tlatelolco area, once a major religious and political centre for the Aztec elite.

Since the discovery of another pyramid at the site 15 years ago, historians have thought Tlatelolco was founded by the Aztecs in 1325, the same year as the twin city of Tenochtitlan nearby, the capital of the Aztec empire, which the Spanish razed in 1521 to found Mexico City, conquering the Aztecs.

The pyramid, found last month as part of an investigation begun in August, could have been built in 1100 or 1200, signaling the Aztecs began to develop their civilization in the mountains of central Mexico earlier than believed.

“We have found the stairs of this, much older pyramid. The (Aztec) timeline is going to need to be revised,” archaeologist Patricia Ledesma said at the site on Thursday.

Tlatelolco, visited by thousands of tourists for its pre-Hispanic ruins and colonial-era Spanish church and convent, is also infamous for the 1968 massacre of leftist students by state security forces there, days before Mexico hosted the Olympic Games.

Ledesma and the archaeological group’s coordinator, Salvador Guilliem, said they will continue to dig and study the area next year to get a better idea of the pyramid’s size and age.

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The archeologists also have detected a sculpture that could be of the Aztec rain god Tlaloc, or of the god of the sky and earth Tezcatlipoca.

In addition, the dig has turned up five skulls and a series of rooms near the pyramid that could date from 1431.

“What we hope to find soon should tell us much more about the society of Tlatelolco,” said Ledesma.

Mexico City is littered with pre-Hispanic ruins. In August, archeologists in the city’s crime-ridden Iztapalapa district unearthed what they believe may be the main pyramid of Tenochtitlan.

The Aztecs, a warlike and religious people who built monumental works and are credited with inventing chocolate, ruled an empire stretching from the Gulf of Mexico to the Pacific Ocean and encompassing much of modern-day central Mexico.