An important question that remains to be answered is what language or languages were spoken by the people and rulers of the empire of Teotihuacan. During the first part of the Classic period Teotihuacan achieved dominance over central Mexico and far into the Maya area. Possible candidates for the language of Teotihuacan have been Nahuatl, Totonac or Mixe–Zoque. Terrence Kaufman has argued that Nahuatl is an unlikely candidate because Proto-Nahuan did not enter Mesoamerica until around the time of the fall of Teotihuacan (c. 600 AD), and that Totonac or Mixe–Zoque are likely candidates because many Mesoamerican languages have borrowed from these two languages during the Classic period. Others find Mixe–Zoque an unlikely candidate because no current Mixe–Zoque settlements are found in central Mexico. Around 500–600 CE a new language family entered Mesoamerica when speakers of Proto-Nahuan, a southern Uto-Aztecan language, moved south into central Mexico. Their arrival, which coincides with the decline of Teotihuacan and a period of general turmoil and mass migration in Mesoamerica, has led scientists to speculate that they might have been involved somehow in the fall of the Teotihuacan empire.