Stone tools indicate that Aztalan was occupied off and on for at least 10,000 years. A substantial Late Woodland village stood here for many years before the arrival of Mississippian people, who built and inhabited Aztalan from about 1100 to 1250. Evidence indicates that the Late Woodland people continued to reside at the site, interacting and perhaps living with the Mississippian newcomers. Sometime after A.D. 1250, the people of Aztalan appear to have abruptly left, but the reasons are unclear. There is no evidence that they simply moved to other locations in the region. Cahokia itself seems to have been declining about this time, and the people of Aztalan may have moved back to Cahokia or other Mississippian villages, either of their own accord or after being pushed out by others. The reasons for abandoning Aztalan remain an enduring mystery.