CHICAGO -- It was the ancient version of a last stand: Twelve clay bullets lined up and ready to be shot from slings in a desperate attempt to stop fierce invaders who soon would reduce much of the city to rubble.

The discovery was made in the ruins of Hamoukar, an ancient settlement in northeastern Syria located just miles from the border with Iraq.

Thought to be one of the world's earliest cities and located in northern Mesopotamia between the Tigris and Euphrates rivers, it is the site of joint excavations by the University of Chicago and the Syrian Department of Antiquities.

Excavations have been going on at the site since 1999, but in digs conducted this past fall, researchers uncovered new evidence of the city's end and more clues about how urban life there may have begun. The University of Chicago was to announce the findings Tuesday.

The site is so close to Iraq that Clemens Reichel, the American co- director of the expedition, has seen explosions on the other side of the border.

"It's somewhat surreal. We're not living in a vacuum there. We know exactly what's happening across the border," Reichel said. "But working in Syria is like working in the eye of the storm. It's very peaceful to work there. Practically no problems."

The site was anything but peaceful in approximately 3,500 B.C. The archaeologists have previously detailed how they believe Hamoukar's independence was ended by a battle that caused its buildings and walls to collapse and burn.

This past fall, the team found more traces of that battle. For example, there was a shallow pit containing a water basin normally used to soften clay sealings for reuse. The clay sealings were used on bags, jars and baskets to help ensure that the valuables or food inside had not been tampered with.

But along this basin, the researchers found neatly lined up along its edge 12 "sling bullets," oval-shaped weapons made of clay that were fired using slings. More than 1,000 of the bullets were found in debris of collapsed walls in 2005.

Reichel theorizes someone who usually worked with the clay sealings was trying to contribute to the war effort and fashioned bullets from the clay instead.

"You imagine the despair the people were in. They were using everything they could to throw back at the attacker," he said. "It looks like a desperate last attempt."

But the roof collapsed before the bullets could be used, and the researchers believe they were the first to see the scene since that fateful day.

"It's the content of 5,500 years ago no one has seen. There's an element of eeriness _ almost a sacred element _ when you do this," said Reichel, a research associate at the university's Oriental Institute.

As for the identity of the invaders, the researchers point to debris that indicates if members of the Uruk culture of southern Mesopotamia weren't the ones attacking, they certainly swooped in immediately afterward and took over the city. Either way, they were probably on a quest for the region's raw materials.

Elsewhere at the site, archaeologists believe they've found clues to why urban life began at Hamoukar.

A massive area, the size of a golf course, is scattered with thousands of pieces of obsidian, a type of rock used to produce tools and weapons. It also contains debris that "tells us that they are not just using these tools here, they are making them here," Salam al-Kuntar, the Syrian co-director of the expedition, said in a statement.

Using pottery fragments for dating purposes, the researchers theorize the area could have been a place where obsidian tools were produced hundreds of years before the ferocious battle.

The discovery could also help explain how civilizations developed in different regions of the Fertile Crescent, Reichel said.

It is accepted that in the south, urban society developed in response to the need for organized labor to support the irrigation-based agriculture. The findings from Hamoukar _ which was on a key trade route linking modern-day Turkey to southern Mesopotamia _ suggest that civilization could have developed there to tap into the market for mass-produced goods (such as obsidian tools).

Guillermo Algaze, a professor of anthropology at the University of California at San Diego, has researched Mesopotamian archaeology and early civilizations. He follows the findings at Hamoukar, but did not participate in the dig.

He said the existence of Hamoukar and the nearby Syrian city of Tell Brak prove that early development of Mesopotamia occurred independently in the north and south, which is contrary to traditional scholastic belief. Previously, civilization in the north of the region was thought to have developed under the influence of urban areas in the south.

Still, the outcome of the battle at Hamoukar in 3,500 B.C. helped change the trajectory of the region, with southern Mesopotamia becoming the dominant force, home to ancient kingdoms such as Babylonia.