Physical traces of ethnic cleansing that took place in the early 800s suggest the massacre was an inside job.

http://news.discovery.com/archaeology/genocide-native-americans-ethnic-cleansing.html

* A massive deposit of mutilated and processed human remains has been found in the American Southwest.
* The remains and other artifacts at the site, Sacred Ridge in Colorado, indicate ethnic cleansing took place there in the early ninth century.
* The genocide likely occurred due to conflict between different Anasazi Ancestral Puebloan ethnic groups.

The source article (needs subscription or library access):
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WH6-50XTT73-1&_user=10&_coverDate=09%2F03%2F2010&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=39e42778505d98d6ba96be3fd67d138c&searchtype=a