At least a third of all people living in the ancient American city of Cahokia were immigrants, a new study has found.

Cahokia, situated in near Mississippi river, was the largest pre-Columbian city in North America. The Cahokia Mounds in Illinois are listed as a World Heritage site in the United States.

The ancient urban center near modern-day St. Louis was a melting pot of cultures and had people coming from the Midwest and even from Great Lakes and Gulf Coastal regions, according to Livescience.

The study, conducted by researchers at University of Illinois and colleagues, challenges the idea that Cahokia had a homogenous population derived from nearby regions.

"Increasingly archaeologists are realizing that Cahokia at AD 1100 was very likely an urban center with as many as 20,000 inhabitants," Thomas Emerson at University of Illinois and an author of the study, said in a news release. "Such early centers around the world grow by immigration, not by birthrate."

In the study, researchers looked at strontium sample ratios in 113 teeth from 87 people buried in the ancient city. They also analyzed strontium levels in small mammals in the same region.

Strontium isotopes in soil, plant and skeletal remains can help researchers estimate the age of fossilized specimens.

 "Teeth retain the isotopic signature of an individual's diet at various periods of life depending on the tooth type sampled, ranging from in utero to approximately 16 years of age," the researchers wrote.

The strontium isotope in teeth can be matched with the strontium signature of the place of burial to determine whether or not the person was from the same area.

Analysis of teeth obtained from different parts of the city showed that one in three inhabitants of Cahokia were immigrants. The city was a melting-pot of cultures throughout its lifetime, which researchers estimate to be from AD 1050 through the early 1300s.

The study is published in the Journal of Archaeological Research.

The sprawling North American city had an abrupt decline. According to a previous research, a catastrophic flood might have destroyed the city.